For an older carbed GM V8 I wouldn't spend the extra $$$ on a painless harness unless the YL harness is totally messed up. FI is different, but with a carb you only need to hook up the alt and battery correctly and the HEI.
It will be easier if you have an early YJ that was carbed, but I would go buy a good manuel for both and work it out from there. The alternator should hook up just about the same, in fact you can probably run the YJ alt on the GM V8.
For pulling power to run the HEI, there is a 10 gauge red wire with white tracer that goes to the ign box on the early YJs that is a good source for the HEI. Tap into that and you should be good to go. I cant tell you where to go for power on the newer YJ's, but it should be pretty easy without having to mess with FI.
For gauges you can go aftermarket mechanical or simply install the Jeep senders into the GM block and use the YJ harness.
Oh... welcome to the BBS, stick around and search past posts... this place is full of good info.