Installing a 235 will help with pulling power, but not top end in any measurable way. The problem is the rear axle ratio. Most 3600's (3/4 ton) have a 4.56:1 rear end gear. There is a very scarce 4.11 center section that was used in a similar application in the mid-1960's that's a bolt-in swap, but they're scarce and expensive when you find one. Nobody makes replacement gears for that housing with a higher ratio than 4.11. You could either install a different rear end with a better selection of ratios, such as the GM corporate 14 bolt, or install a 5 speed overdrive trans, but that requires some pretty extensive modifications. If you go too high on the rear end ratio, you'll get out of the torque band of the 216, and bog down on every hill.
Once you get the truck up to highway speed, then you need to consider a few other little details- - - -like steering and stopping. At the very least, you'll need to bring the steering, suspension, and brakes back to brand new original condition. Otherwise, you're going to have a rolling deathtrap on your hands that's a hazard to your safety, and that of everybody who shares the road with you. Good luck!
Jerry
