I have an '81 C3 that my son bought for a song a couple of years ago as a project car for us. It has the old 3 speed automatic in it. We've souped the engine and in teaching him that it doesn't matter what you do to the engine to soup it up, it's not going anywhere without the right tranny and maybe different gears in the rear. Does anyboby have a suggestion as to a tranny change we could make to get some speed pasat 85mph? Obviously, now days, we'd want some fuel economy also.  Thanks!  

Might want to try Keisler transmissions. http://www.keislerauto.com
They have a few options available that should slide in for you.  

Hello.
A 700R4 from a 1982 TO 92 Corvette is what you need. All you need is the transmission cross member, drive shaft and tail shaft housing from a 1982 Corvette, And then any 700R4 transmission will work, Just bolt it in, The over drive will make a big difference in fuel economy.
David Steele.  

Thank you for the info. I'll check it out.