I bought a 1971 454" Corvette about a year ago with the intention of restoring it to a "daily driver" condition. It had a TH400 transmission in it and I quickly found out it's highway fuel mileage was only 10-12 miles per gallon. After putting about 300 miles on it I decided to replace the Th400 with a more modern TH700R4. I bought a used TH700R4 from my local wrecking yard then tore it apart. I upped the pressure, put in a performance 3-4 clutch pack in it, and added Fairbanks servos. I had to have my drive shaft shortened 1-1/2" and have a rear end "bearing strap" style of yoke welded on the front. Altogether the drive shaft alteration cost me $193 but that also included two new u-joints. And to make the swap easier I put a new tail housing on it that had a rear mount pad; allowing me to set it on my existing cross member by only slotting my cross member 3/4". I had to fabricate a new bracket for my throttle cable and the TH700R4's TV cable and I also had to rework my parking brake cable pulley bracket. In the end the swap was pretty easy and I ended up getting 18+ miles per gallon on the open road thanks to the TH700R4's 30% overdrive in 4th gear.
If you have a C3 with a TH350 and you drive it a lot I'd suggest doing what I did. With the TH700R4 my '71 cruises at 60 mph at only 1750 rpm; rather than the 2400 rpm it used to spin.
