This is the infamous GM 700R4 4 speed automatic transmission that GM put in everything from regular pickups to Corvettes. In this mod there is actually 2 different transmissions, one is the regular 700R4 and the other is a 8 speed Gear Vendors unit shown here https://www.gearvendors.com/hrgm4s.html
My 1990 Chevrolet suburban 350 silverado model has the Gear Vendors unit installed and is really nice to drive.
So far I've made this trans available in a few vehicles and I will be adding to more of them soon! https://en.wikipedia.org/wiki/Turbo-Hydramatic
I've added this trans to the D Series, Roamer, Grand Marshal, Van, and Moonhawk for now, more to come. Thanks