THat tranny swap won't be an upgrade, it'll be a down grade. Sure you get a better first gear but in o/d you'll have a 2.10 rear gear. It'll be constantly shifting down, any gas mileage you have will drop like a rock. You need a 3.73 in the back for it to run right.
Rebuild the T350 and use a 2400 stall convertor for a little more oomph off the line.