was debating on doing the same with my 85 D150, that I am currently reviving.... needs Lean Burn deleted anyway.
Which I used to do fairly regularly ~20 years ago but not so much lately.... the wiring on this truck is nothing short of a train wreck. I have to basically rebuild the wiring harness anyways due to past owner "handiwork".
I got the best harness I could find out at my local boneyard, happened to be off of a 84 D350 that had a "heavy duty emissions package" 360 in it.... it too has a couple of problems but way better to start with than the original.
there was another 84 with "heavy duty 360" right in front of the one I got the harness from, was a little worse than the one that I ended up with but either are way better than the original.... and the "new" harness is already 4 pin factory Mopar ignition equipped, which saves me a little bit (I admit, "not much") work vs the original harness would, already even if both harnesses were in good shape. I scouted Ebay for new old stock Mopar replacement ECU from a reputable company "from back then".... I bought a few, and judging by shelf wear, Id bet all that I have are 30+ years old even though never used.....
though when I undo a Lean Burn I unwrap the harnesses and pull out any "extra" now-unneeded wires and plugs for a clean install, not needed I know....
I still might do as you are when my new motor is ready to go in..... whenever that is.... the original still runs so not in a hurry/ just want a "Tim Taylor" (remember "Home improvement" TV show?) "more power" infusion.... while staying with a slant.