833OD into 78 D100 questions

The long tailshaft tranny is the correct for your pickup. When the front lever on the tranny is flipped tranny will be in the 1:1 gear ratio when the lever is in the thrid gear position and will be in the OD ratio with the lever in forth.

I pulled an A833OD from a D100 for my cuda an the z-bar from the car mated to the OD bell housing with no issues (this was a /6 setup too).

I don't recall a hump on the floor of the truck but the boot had a plastic base that fit the contour of the floor. The shifter mounted to a play that positioned the shifter above the tranny an slightly behind the front mount on the tranny.

You will likely find the slip yoke of your 3sp won't fit the A833OD.