With a 3.27 rear end a o-drive would be worthless with a 250 engine unless you were running some silly low pro tires.
What wrenchbender says.
To get back to the NV4500 that was mentioned earlier... If you use the 94-97 dodge 2wd version it will push the shifter back about 6 1/2 inches from where the sm420 shifts if you use an adapter plate. You could run a different bellhousing behind the 250 and mount the gm version of the Nv4500, but you would have to convert an electronic signal for the spedometer.
I would rule the T5 out for a 3/4 ton truck. I don't believe that transmission is built strong enough to do the work.
Overdrive transmissions work pretty well with lower gears. 3.73 gears with a 27% overdrive would effectively become 2.73 gears. 4.11 gears would become 3.00.
