I've seen a post from someone putting a 4L80e in a Vette somewhere. The physical size of the tranny was going to require some floorpan mods. I'm sorry I don't recall where it was I saw it. Is there a 6 speed auto that would have a chance of fitting? I've got an Allison 5 speed in my truck, and that thing is HUGE- especially when you start looking at the tunnel on a C3. And they do come in a 6 speed. The 4L80e has a lot of torque capacity, AND a lockup convertor, but they're pretty heavy too.
I'm pretty sure this is wishfull thinking. I have a 700R I may rebuild as a backup plan. I was wanting a lil better fuel milage for trips. I am now just getting the beast road worthy but am eyeing future improvements.
Most of the tricky wrench work is done by my step-dad and I'm there for parts funding and easy stuff.
A $2500 budget should be an easy fit for a bumped up 700.
Ebay can get you the rebuild kit for around $250
Shift kit annother $150 or so
SOme other hot rod parts for the inside annother couple hundred
Torq Converter $200
you got fab skills and tools, custom crossmember ~$50
Get the drive shaft cut to fit and balanced, don't know the $ for that.
you can easily do this part for under $1000
Tranny not that hard to rebuild, I just did mine with a book and a video.