[Mark Yacavone]

It appear that this car uses the 4T60-e o.d. trans. It has a lower first than the older T-125 3 speed. There is a 3.90 something final drive out there, whereas you can only make a 3.73 for a 125.
You don't need the o.d. of course. I'd think about retro fitting a T 125 , seeing those cars came with them earlier.
Otherwise , the supercharged Bonneville guys can help you with shift programming.
If you study the solenoid wiring, you'll see that the shifts could be controlled with a bank of toggle switches.

If you're going to do a semi-serious race only deal, I'd be glad to help you. It ought to run .5-6 under anyway.
I don't believe it'll run under as a street car.