Need guidance on getting 340 running again

Wiring: I would not assume the engine harness is OK just because maybe ?? it looks that way. (Don't know what you have inspected, how closely, or if you have run continuity tests

Unfortunately, these girls are VERY VERY poorly fused. It is VERY likely with a wiring problem, that one wire got hot and melted/ welded to others in the harness. If you are going to try and repair the harness, pull it out, lay it out, unwrap parts of it at first, and inspect for heat/ melt damage.