Do you want it to be automatic or manual? Are you going to be traveling on the street more than the trail? Will it see more hi-way use or in town use?
The Chevy Turbo 350 3 speed auto is a stout
Transmission the will handle the power and abuse of the SBC, plus it's compact enough to give you decent drive shaft lengths.
The
GM Turbo 400 is beefier than the 350, and a hair longer, it also has an electronic kickdown solenoid so you don't need a kickdown linkage.
The 700R4 is also a stout
Transmission and offers an
Overdrive gear for cruising down the hi-way at a more comfortable RPM, however, it is longer and has been known to have clearance issues with the front drive shaft.
All the Chevy trannies have the beauty of bolting to the SBC with no expensive adapters, as well as strong aftermarket support for converters, valve bodies, vacuum modulators etc.
With manual trannies you have basically the indestructable SM465 cast-iron 4 speed truck
Transmission , with a great granny gear for crawling, but no 5th gear for cruising.
And the NV4500 cast-iron 5 speed truck
Transmission , again with a decent granny gear, but also a 5th for cruising at decent RPM's.