Where there is an SM420 a T5 can replace it, and it is bolt in with the proper parts, clutch disc being the most difficult to get right, everything else should work as is. This has been covered any times here, try a search.
If you have a 250, then does that open you up to different bell-housings and any transmission that was as behind a small v8? I don't know..
What ratio is in the rear you have to swap?
Use this site to figure speeds and ratios to get what you want
http://www.onlineconversion.com/bigger_tires.htm I like the last one on the page
Do some checking, but I think you will find that the 250 likes to be run about 2500 rpm at cruising speed (you say 60-70) when empty and on the highway. So that speed, or whatever you find out is best, is what you are shooting for when you plug in the numbers into the calculator link above, the OD ratio of the trans, or direct drive (1; like for the SM420), rear axle ratio, and tire outside diameter.
Grigg
