The main difference in the '77 vs the '78 is the ignition components. For 1978, Jeep switched from the Prestolite BID (breakerless inductive discharge) system to the Motorcraft Duraspark system. The Prestolite stuff is a problem; it is both an orphan technology and quite unreliable. The Motorcraft ignition is way more reliable, and uses a trigger system widely used even today (variable reluctance or VR). You can mix and match components with the Motorcraft distributor between several other manufacturers that use the VR technology. Not so with Prestolite - you're stuck with the repop BID stuff, if available.
Suggest you get rid of the Prestolite stuff, and go with Motorcraft (my preference) or one of the aftermarket ignitions. The '78 harness is set up for Motorcraft stuff. You can also revert to pre-74 Delco points ignition, if that appeals to you.
I would get the wiring diagrams for '77 and '78 and compare. The bulkhead connector in the firewall should be the same, though some wire locations will be different. I would print out the '77 and '78 diagrams double-size and trace what's different with colored pencils. Map the bulkhead connection numbers between '77 and '78.
Both the '77 and '78 TSMs are online, free to read and download, on the Tom COllins site. The wiring diagrams are also there, in a separate section. Suggest you look at both for the diagrams, since the image quality could be better in one or the other.
https://oljeep.com/gw/elec/GW_wiring.html
https://oljeep.com/edge_parts_man.html
TSM.png
I believe you will find that the differences '77 to '78 mostly are limited to the ignition. The '78 will have a resistance wire to the coil that is not present with the Prestolite system.
A '78 switches to the Delco SI alternator for V8 models, as I recall. You could adapt to the '77 Motorcraft alternator, but the Delco is a significant upgrade, and I'd keep it.
The starting circuit will be slightly different, with the addition of both a wire that bypasses the resistance wire at starting, and a connection to the Motorcraft module that retards the spark while starting for easier starts.
JMO - you seem to have plenty to work with here, and an opportunity to get this Jeep on the road with a bit of research, planning and attentive rewiring. The Painless option wastes your money (expensive) and effort (quite a lot). Nothing wrong with the '78 harness, and it's miles and miles closer to complete and running than any hot rod harness like Painless will be.
Suggest you learn something about the 401 before you consider rebuild. If you have a virgin block, that's a great asset. The 401 pushes the bore diameter to its limit for its block size, and the cylinder walls are thin compared to your typical grey iron USA block. If you decide to rebuild, suggest you do your research and have the block sonic checked for core shift before you bore it oversized. A seasoned engine is a reliable engine - have you checked compression or looked at the hot idle oil pressure? IMO it would be foolish to rebuild any engine that's in good condition just because of its age.
You're in a different universe with the diesel conversion. How much experience do you have with projects like this?
hth! TIm
You do not have the required permissions to view the files attached to this post.