New guy with some questions

[Wildmani]

Just got a '79 LeMans project and had a few topic related questions while I'm in the planning stages. I've browsed through the board and got some ideas already, hopefully you guys can bear with me in case this post gets long. The car currently has a 267 V8 with a 3 spd auto (assuming it's a TH200?), I haven't gotten around to checking the rear end codes but I'm guessing it's a 7.5" non posi rear. A friend of mine's dad has a factory 300hp 350 V8 and a TH400 out of a '69 Chev wagon that I'm getting for free so that's what I plan on swapping in. My goal is to have around 325-350hp when I'm done and I have a few questions about this swap and what else I'll need to make it happen.

1) I want to get an 8.5" posi rear with 3.73 or 3.42 gears. I know that Monte Carlo SS, 442, and GN's would probably have something like that but are there any other donor cars I could be looking at that would have the same rear end without having to modify mounting points, etc.? I want something I can bolt right in. I used to have an '81 Z28 that was up around 300hp that had an 8.5" posi with 3:42 gears and had no problems so I'm guessing strength won't be an issue?

2) Will the TH400 bolt in wihout too many mods? I figure I'll have to do something with the rear mount but what about linkage? I'm also assuming I'll be needing a new driveshaft due to length or # of splines compared to the stock one?

3) How much of a pain would it be to change it from column shift to floor shift? Obviously I'll need to cut the floor, but would the linkage be an issue? It's a full bench seat now but I'd like to switch to buckets and console with a floor shift. My guess would be that I could use a lot of parts from a GP (linkage, console, seats, etc.) but I'm not sure.

I was lucky enough with my Z28 that I never had to swap transmission types or rear end because what it had was fine. The engine rebuild I'm comfortable with but I don't have a lot of experience in parts swapping like this so any advice would be appreciated. Thanks.

 

[87BlackSheep]

Instead of checking the codes for your rear end have a look at the diff itself. there are two tabs at the 4 and 8 o'clock position. The 7.5 has "fangs" whereas the 8.5 has more bulky squared off tabs.
8.5" Differential

7.5" Differential

Now I'm not sure what suspension your LeMans has in the rear, but if your not afraid of some fab work you could also find 8.5's in most full size pickups (ie. Chev/GMC 1500's), so I'd go scrapyard huntin. 8.5's are really sought after so it's gonna be kinda hard to find one, esp. one that'll bolt right in to your suspension, but they are out there. But it's pretty awesome you got a Th400, that's a great transmission. But as far as the floor shifter and all that, I'm really not too familiar with the linkage and other mods you'd have to consider. I'd just slap on a B&M or Hurst shifter. Most are cable-driven but are easy to install.

 

[Phoenyx]

1- those are the only car's that will have bolt in 8.5s. Unless you find one that's been swapped in. But you can also buy aftermarket bolt in 9"s and 12 bolts.

2-For the transmission you will need a TH400 mount, there are several aftermarket ones you can buy. And yes I'm pretty sure the driveshaft will have to be shortened (but not 100% on that).

3- As for linkage, with the column it might be a pain in the but. But since you want to go to floor sh*t it should be easy. The floor shifters use a cable, so you're plan is actually the easy way lol. A small hole for the cable to go through and mount the shifter to the floor.

For your buckets and console any g-body car should work. There are a few people that have put an aftermarket shifter with their bench on here, but I'm not sure what model.

 

[pontiacgp]

if you are looking for a bit more power and an easier fit you might want to consider a TH350, they are lighter and takes less HP to turn them

 

[Phoenyx]

And also, I like the TH350s first gear better.

 

[Wildmani]

Thanks! I'd consider going with any transmission. I had a TH350 in my Z28 and had no issues, but the price is right with the TH400, getting it for nothing lol! I was under the assumption that the TH400 was a more heavy duty transmission. On a side note, should I be getting a stall converter or would the stock one be ok?

Got under the car today to have a look, it's definetely a 7.5" and decoding it says it's a semi with a 2:56 gear. Guess I'm going to have to start hunting.

 

[Phoenyx]

The 400 is a great transmission, don't get me wrong. And getting got free makes it that much better. But the money you saved on getting it for free will have to go into a new mount and a drive shaft (probably cheaper that anyway lol).

A stock converter should be good for that. And for your rear, it also depends on what you have for tires and what you plan to do with the car. If you're running slick at the drag strip every weekend, well yeah, start looking for a better rear fast. But if you're just running street tires on the street, the 7.5 should hold up, for a while. I would still start looking for something else. And maybe with better gears, it will make all the difference in the world.

Mu Malibu had 2.simthin' gears when I got it. With the stock 305 the car was pretty gutless. Then after I had my 3.73s installed it felt like a whole different car. Beat quite a few 5 litre Mustangs light to light with my old 305, all stock except duals.

 

[pontiacgp]

wild, your right about the 400 being more heavy duty than the 350. The way I heard the story was the 400 was built for the big block's torque and takes 40 hp to turn it. The 350 takes 35 hp and it will easily handle the power your looking for and be an easier swap

 

[Blake442]

TH350 will be a direct bolt in.
The TH400 will require a new crossmember, a shortened driveshaft, as well as a new slip yoke.

Don't waste your time with trying to find an 8.5"...
A 7.5" will handle the 300-350hp you plan on making just fine on the street.
Just swap out the gears to whatever you want and add a good posi-carrier.

 

[bawazir]

B)B) B)B) i need a posi rear end