Join Date: Jan 2019
Location: Sedgwick, Kansas
C6 is a much heavier transmission, and while functionally identical to the C4, it steals a lot more power. It's a lousy choice for a smallblock. The C4 is ubiquitous. It came in cars, trucks, vans - anything with a 6 banger all the way to the 351, from mid 60s all the way into the 80s.
Any salvage yard that has older cars will have a few. It would be an excellent choice, even if it needs a rebuild.
If you don't care as much about originality, and want to upgrade the rear gears too, you could go to an AOD - or better yet, a 4R70W. There will be some other considerations with these 4 speed overdrive transmissions. They both require a different transmission mount from the C4, and they may require a different length of driveshaft. The 4R70W is better in most every way, with a lower 1st and 2nd gear, and better durability. It does require an electronic controller which will set you back about $800 or so. It's an updated version of the AOD, and features a lot of upgrades that affect reliability and shift quality.
The C4 is a fantastic transmission. It's tough, simple, lightweight, with good shift quality and it's very easy to rebuild. But in the end, it's just a 3 speed.
If you're interested in maintaining originality, or just want to keep things simple and easy, take the time to find a C4. They're around if you look.
If you're more interested in hot performance while maintaining economy, and don't mind spending a lot more, step up to the 4R70W, and enjoy better mileage along with hotter performance. Running 3:50-4:10 rear gears with an overdrive transmission is pretty sweet. Running an overdrive transmission with 2.79 gears really lugs things when you're in 4th.
Last edited by Grimbrand; 04-23-2019 at 09:35 PM.