I did same as you want with my first TF, a 57 1/2 ton with 4.10s in it. The old 235 just didn't have enough poop to run more than 55 or so on the highway.
In my local junkyard I found a 66 1/2 ton pickup with 327 and sm420 4 speed. All stock stuff, and worn out too, but better than I had. I bought the replacement SB mounts, dropped it all in and drove it for a couple of years.
Also- Install the tallest rear tires you can without causing problems. That will effectively reduce your rear gear ratio.
Devin
