Like Jocko I am jumping ahead, except to the six speed. If you use a T56 and the bench seat you need to relocate the shifter in the transmission to a forward location.
http://67-72chevytrucks.com/vboard/s...d.php?t=300892
With your low hump floor the shifter housing will have to stick up into the cab. But it only takes a 3-1/2" hole saw which gives 1/4" clearance all around.
I used an old shifter rod out of an original 4 speed floor shift and modified the end to match the T56. Now it looks like the old original 4 speed floor shifter. I used the original shifter ball. It takes a big boot to cover the shifter housing. I used one for a Peterbuilt tractor.
With the 2:1 overdrive in sixth, it really cruises the highway nicely.
TR