4.3, 5-Speed swap

That shouldn't have much problems doing it. The shifter will be centered in the transmission tunnel unlike the factory 3 & 4 speeds. Years back on a G body mailing list they talked about it being an easy swap. The 5 speed should be a Borg Warner T5 & you'll need a BOP bell housing for the Buick. The plus would be a hydraulic clutch & not trying to locate the bell housing for our cars. You'll have to get a T5 shifter from Hurst to replace the S10's O.E. unit.

The S10 used a T5 trans built by Borg-Warner, now owned by Tremec. 4.3L Chevy is a 350 with #3 and #4 cylinders lopped off (3.8L/229 Chevy was a 305 with 3/4 missing). Serpentine accessories and bellhousing/flywheel interchange between 4.3 and the 305/350.
3.8L Buick uses an entirely different bellhousing and the flywheels are going to be pretty rare - not sure if a '70s Buick 350 flywheel might work. Some early 70's Jeeps used the 3.8.
The easy(er)-to-find G-body '606' bellhousings only fit a 12.5" 153-tooth flywheel that takes a 10.5" clutch disc. This is what was used in the 80s Camaros.
The S10 T5 almost exclusively came with a 4.03 first gear/0.86 overdrive. The F-bodies had better choices - 2.95 or 3.35. Only the '93 S10 models were 'World Class'- set up with the better bearings to handle more torque. But, GM for an unexplainable reason, used a Ford bolt pattern on the front of the trans - if you go this way, you must use the S-10 4.3L bellhousing it came with - hydraulic clutch system.
The 4.3 L motor does not have a lot lot of performance parts available. You could try to use the factory TBI system, or locate a small 4-BBl carb for it. I think Edelbrock may have made a carb intake for it.
If you want fuel economy and a manual trans, find a running 3.8L Camaro or Firebird, 1993-1996 and take EVERYTHING fan belt to yoke. This T5 had a 3.35 first gear like many of the Mustangs. The shifter from an F-body comes up in a good place in the floor on a G-body, right about where the floor shift comes up out of the console.

Sorry for the stream of conscious. The range of options is pretty wide and generally good only if you have just the right luck.

I have one of the Iraqi Malibus that GM of Canada built for shipment to Iraq in 1981. These cars were equipped with the Chev 229 V6 and the 3 speed Saginaw manual transmission. The plan for this car (when I get around to it), is to swap in a late '80s 4.3 V6, and a 5 speed from an '80s S10. One of the advantages of using this car, is that its already equipped with all of the manual transmission bits & pieces, so there's no hunting around for those parts, and the engine mounts are already set up for a 90 degree Chev V6. I've done some reading on other forums, where people say that the input shaft on the S10 5 speed is approximately 5/8"-3/4" too long to fit properly in the 90 degree Chev bellhousing, and into the crankshaft on either the 90 degree Chev V6, or the Chev small block V8. In most cases where this issue comes up, the person posting just cuts the appropriate amount of material off the input shaft of the S10 transmission, and this has solved their problem. There is a company called Hamilton Intakes, that manufactures a spacer plate that installs between the bellhousing and the S10 T5, that spaces the 5 speed back the proper amount to correct the issue with the length of the input shaft on the S10 trans. I haven't had any personal experience with this just yet, so I can't confirm this issue. The shifter on the S10 transmission should come through the floor in approximately the same location as the shifter for the 3 speed, but once again, I can't verify that just yet.
I want to keep this car carbureted, so I tracked down a factory cast iron GM 4 barrel intake from an '85-'86 light duty truck (here in Ontario, emission controls are required by law), and I've got the factory tubular exhaust manifolds for the 4.3, that GM used in the mid to late '80s. The engine block for the 4.3 that I have has the mounting pad for the mechanical fuel pump, but the bolts holes for mounting the pump aren't drilled and tapped, and the hole for the fuel pump push rod has to be drilled (spoke to a local machine shop guy, and he tells me that since some 4.3s were used for marine engines with carburetors, that this shouldn't be a problem). The later 4.3 engines have the provisions for a newer style serpentine belt drive system, so I'm going to try that as well.
The Malibu chassis will accept most, if not all, of the popular suspension and brake upgrades for the A/G body cars, and I'm figuring that since there will be less weight on the front wheels, that handling should be a bit better than a V8 equipped car. The goal for the 4.3 will be something around 200-225HP. This should be within the limits of the S10 trans. I have an '84 Monte SS, and the 305HO in it was rated at 180HP from the factory, so I think that a 200HP goal for a 4.3 is realistic, and should provide a car that's fun to drive, and not too bad, in terms of fuel economy.
I don't want to build a race car, just something that's fun to drive, and a bit different.
If you decide to try something similiar, please keep us updated with your progress. Good luck.

You can't get to 200hp on the 4.3L without using the TBI setup or wasting thousands of more dollars on machine work that will only get you about the same hp you could have gotten stock by just using a TBI system for cheaper...

PS going with the TBI setup also makes it super easy to install AC and add the blazer hvac controls out of sight under the ash tray (or use the ash tray bracket to mount it faced out)

The TBI setup is extremely reliable and the best way to push a 4.3 v6 by miles... It will cost you a few mpg in the long run but it is significantly more fun to drive on a daily basis. Both of my 1st cars were box blazer w TBI and when I tried the 4.3 w/ carb I found it weak and disappointing in comparison. Didn't buy the carb setup blazer b/c it felt like a steam engine in comparison to my TBI blazers off the line... After I put a chip on my TBI the engine was incredible and I was even trying to race stock hondas / acuras/ mustangs - smashed all of them easily. Downsides to the TBI? You won't get over like 17mpg on best day and it does like to fail emissions test. Those are really the only two downsides to it. The 4.3 w/ TBI will have you imagining the Chevrolet is a Porsche until you try to turn too fast and snap out of it lol. One thing I would note about it is I tried one w/ dual exhaust and found it not only less powerful than mine but the exhaust note was obnoxiously loud and irritating. Don't put duals on a 4.3 v6 or it sounds horrible bad.

whoever told you that you cant have the TBI setup connected to the 5 speed same time is wrong b/c it came stock on some early 90s silverados namely Stepside came w/ 4.3 or 5.7 tbi w/ 5speed. If you put the TBI behind the 5 speed you could have speed and mileage same time! The carb would offer more like 21 mpg than 17 though

http://www.youtube.com/watch?v=7CwZ_yKzVVo

heres another dude having a similar experience as me w the 4.3 TBI same thing he got it cheap - didnt even think it would be 'fast' and now he has collected a bunch of haters. All you need is alum wheel so its not having to work so hard and the 4.3 TBI can be really fast esp w air intake