My dad and I recently put the engine, transmission and manifolds from a totalled '94 Ford Ranger into a '96 Ford Ranger (to replace a cracked block). Despite the fact they were just 2 years apart, they were surprisingly different. Had to transfer all the manifolds and everything on the engine belt (e.g. power steering), because the '96 parts wouldn't mount on the '94 engine. Broke the '94 belt tensioner taking it off and the '96 that replaced it sat at a different angle, meaning we had to find a longer belt to compensate. Rewired the whole thing. (Except for lights) Even the wires that had a 1-to-1 match (about 80% of them) had to be extended or shortened, because the control unit was on the opposite side of the engine. Took about two months of combing through manuals and schematics to get everything wired up right.
Once everything was together... Turned the key and it purred. There were a couple engine codes (I forget what they were, but we determined that it was basically the control unit saying "WTF Is going on!"). Gotta love the feeling of a hack JUST WORKING. Had to replace the drive shaft and a few cosmetic parts, and then drove it 200mi home the next day, costing me 9 gallons of gas. I've been driving it for a month since.
I've heard of more impressive motor replacements, but prior to this, the most difficult repair I'd done is replacing my brake pads.
Also, we broke a chain fall and "2-ton" wince, dropping both motors right after pulling the chassis out from under them. Luckily we had tires set under the good one, so it didn't crack, and neither of us were dumb enough to be underneath. That was fun.
Also, I've learned I like driving a stick.