transmission swap ideas

http://americanpowertrain.com/

I have a 1972 big block car.  472 cubes - mild build making about 500 ft-lbs of torque.  I wanted the manual trans, and at least 5 speeds with an overdrive for easy cruising.  I have used the Richmond 5 speeds and they are bulky and I felt they would require trans tunnel mods to fit the shifter linkage in - more than what the factory 4 speed linkage bulge would allow.  Mine was originally an auto car and it is a 75,000 mile rust free version and I did not want to cut on the trans tunnel.  Tremec  has a reputation for having balky 2-3 shifts and the 6 speed is not going to fit without major surgery.  What I settled on was a T-5 built to A-5 specs with upgraded main shaft to handle 575 ft-lbs.  It is small, and fit like a glove with plenty of exhaust clearance and with the shifter coming out the top required only the appropriate hole in the tunnel.  I bought mine from Stan at Pro-Force Performance in Lawrenceville GA (Atlanta area).  He buys the upgrade parts from Astro and builds them himself.  Great guy to deal with, knows his stuff and will not let a trans leave his shop until it is perfect!  The trans shifts like a hot knife through butter!  I used the Hurst T-5 shifter. Honestly, in 40 years of this hobby, Muncie's, Saginaw's, top-loader's, Doug Nash/Richmond, etc., it is the best shifting trans I have ever experienced.

Have you considered a Gear Vendors OD unit, bolted to the existing tranny?  You can set them up like a splitter, and have 8 forward gears, and 2 in reverse!

The real issue is finding a big block AOD that is reasonably priced. You can go aftermarket and get a seriously strong one, but your charge card will melt.

I wish I had bought Allan's GV when he was selling it. That was a nice one.

I was planning to put an E4OD in my first GTS. It had a 429. I found it would clear with no mods to the tunnel, shifter mod was easy. The only major mod needed was was relocating the trans crossmember rearward about 5 inches and shortening the drive shaft. A properly built E4OD will handle 500-600 hp and loads of torque. You will need a shift computer at a cost of $600, trans is about $1700-$2500 depending who you get it from. The computer is fully programmable and easy to do. I put an E4OD in my 8000lb F350 and it works great!

If you are ok with an auto then that might be the way to go. If you build a big enough motor just put a pretty shallow gear in the back and stick with the dirt cheap and easy C6. Something like a 3:1 rear gear actually isn't that bad on the highway and if you make like 500ft/lbs at the crank it will brake most tires loose pretty easily when you put the pedal down. When big motor torque is involved, traction will be the weak link anyways. Going to a limited slip for me was a huge game changer. I have a mostly stock internals 460 to a stock C6 to a 3.25:1 limited slip rear dif, good rear suspension setup, and above average tires. I can still break both tires free off the line in first gear. I'm making probably 300-400ft/lbs at the motor. I just hate the auto transmission with a passion. Some people love the autos though. I'm sure with modifications it could be better. I just wanted that third pedal. 

C6 is three speed. E4OD and and AOD is 4 speed. C6 is easy and cheaper and would handle good HP.