First off, what engine do you have? If you have a BOP (Buick/Olds/Pontiac) block have fun finding an original bellhousing, they're even more rare than the Chevy ones. You may be able to find a way around it with a safety (racing/SFI/$$$) bellhousing that has multiple bolt patterns though.
Here's a longoing thread about a TKO install in a Malibu... comments over the past 5 years:
http://www.maliburacing.com/forum/viewtopic.php?t=29500
Some info I gathered on swapping a Saginaw, T10 or Muncie into a G-Body:
http://www.maliburacing.com/forum/viewt ... =7&t=89587
Very good info on swap parts if you can't find the correct bellhousing (
t5montecarlo on MalibuRacing.com):
http://www.garage-scene.com/t5-into-ag- ... ousing.htm
In a nutshell, you will need a bellhousing/fork pair that match, and will line up with the geometry of the original pushrod. There are a couple other fork/bellhousing combos you can use, they're outlined at t5montecarlo's site. As I said before though, if you need a BOP bellhousing and can't find one, look through Summit, there's a chance you may find a safety one that will fit the traditional GM trans bolt pattern and will also fit a BOP block. Know that these bellhousings start somewhere in the $350 range and go up.
If you don't already have the rods, you can get better-than-stock pieces
here. They have spherical rod ends that won't wear out like the originals. If they do you can just replace them and not the entire rod.
Far as the Saginaw/Muncie/T10 trans go, all have the same mounting dimensions and share the TH350 crossmember mount point. Most share the same output shaft yoke with a TH350 and use the same driveshaft, a couple have a TH400 yoke.
If you're swapping in a T56, you will need to do one of several things. A)Go with the OE bellhousing which is T56-specific and set up a hydraulic master cylinder - in which case the stock G-Body clutch pedal will need to be replaced with a proper one, a couple people make ones to work properly with the F-Body master cylinder (and maybe use a hydraulic throwout bearing like wagon nut mentioned; you will also need a custom crossmember and driveshaft length), and you will need an LT1 application T56, input shaft on an LS1 T56 is too long, B)go with an LS1 T56, get an adapter plate from someplace like
The Gearbox, bolt it to the stock bellhousing (or if you have to find one that will work with a BOP block like I mentioned before) and use the OE linkage, or C)go with an aftermarket T56 that shares a belllhousing bolt pattern with the Saginaw/Muncie/T10 and use the stock (or similar piece) bellhousing/fork.
T5 transmissions are cast at an angle, so the shifter will be slightly tilted away from you... you will have to make adjustments for the crossmember mount and shifter (some have just bent the shifter arm and just had to reweld the mount pad at an angle), you will also need a custom length driveshaft. There is one bellhousing that will straighten it up and allow use of OE linkage,
here, but you will also need a straight clutch fork, which is listed on t5montecarlo's site.
I think that covers most of it... I'm sure you know you'll have to cut a hole in the floor for the shifter
anakputa mentioned headers. If you have an SBC you can use Hooker 2451s and they will fit with mechanical linkage, it's what I have on mine. I'm not positive what other brands fit. I can say my headers went in without a fight, but you have to be willing to remove both the oil filter and starter, and jack the car up quite a bit on the driver's side (I mean a lot, not just a little more than what you're used to), the header slips right in from the bottom (easier if you have the plugs out as well), the passenger side went in from the top.
Now that two people have posted in the time it took me to type this (and make sure I had all the info, which I still think I may have left a couple things out), here's some more info
*edit*
I originally planned to just post the first three links, but I couldn't leave well enough alone