@Kuper , what part of the country are you from? Just kinda curious if you’re anywhere near me.
Anyway, sorry I didn’t see this sooner. So I don’t really blame you from not wanting to deal with wiring, it’s definitely not for the faint of heart. Unfortunately, what you’re up against with this is the most ideal of the less than ideal situations. On one hand, the wiring on the 88 isn’t going to match up with the wiring for a 4.0. The 89 wouldn’t have the 4.0 unless someone swapped one in. The wiring for the cab and chassis on the 89 would be close to matching up with the engine harness of a 4.0, but there may still be a little splicing.
I’ve been down this road before. I ended up abandoning my efforts to fit a 4.0 in my 88 Bronco II because at the time I tried the project, I didn’t have access to the necessary wiring diagrams. I hadn’t started hacking up the harness so when my 89 needed a bigger motor, I swapped the 4.0 into the 89, which still required some splicing but I had diagrams and it was pretty straightforward anyway, think it was two plugs were similar but not the same, may have just been because of using a Ranger harness in a Bronco II.
That all said, there is a couple tweaks you can give the 2.9 if you want a little more go. Mild cam maybe, definitely free floating rockers, headers, bit of gasket matching or flow porting on the intakes and heads. I started down that road with the 2.9 for my 88 and I was also going to try doing a conversion to a Mass Air Flow system rather than the stock Speed Density that most of them had to see if maybe I could squeeze a little more fuel economy. Electric fan and electric power steering pump would require an alternator upgrade but less drag on the engine…