What to do?

[OneGoofyGuy]

I bought a 1989 Ford Ranger XLT 4wd Manual from an old timer. He said that there's a ohv 4.0 under the hood, and the tranny that's sitting in the cab would hook up to it perfectly. All that he did was sit it under the hood and left it there. He didn't say anything about a engine swap from a 2.9 to a 4.0. The good thing is I've got a 1993 Ford Ranger XLT 4wd Auto that recently the tranny doesn't pull at all anymore, needing another put in. The truck has served a very long life no exaggeration it's got 589,000 miles on it. It became mine when I turned 15 and before that my uncle and my dad drove it before me. I did 1 rebuild on the 4.0 motor, always did maintenance like plugs, wires, oil changes, brakes, etc., and kept fluids into it. I took care of it. The main thing I need help with is I've got good parts to go into the 89 it's got the same engine in it as my 93. I've been reading the entire page on the 4.0 swaps from 2.9. I'm just a little worried something will come up and basically ruin the whole ideal. Please help?