My son just shipped me a manual overdrive 3 speed he had.
Nice thing about this is no changes needed to the crossmember. (replacing my SM420 4 speed) I'll use an aftermarket floor shifter, for now, and someday probably install a standard shift column.
I switched out my 3:93 gears for 3:73's. Havent got it together yet, but with the approx .70 to 1 top gear, I'm hopeful it will work out well for cruising, and should still not be a slug in low gear. I have a stock 350 truck engine with a 4 barrel carb.