When you deal with all things electrical, keep these things in mind.
The body/frame/chassis is the ground.
Over time, many areas that serve as ground path can deteriorate, create a high resistance and all sorts of intermittent and troublesome electrical issues. Add to that, the "new" electronics that are incorporated to 50+ years technology in the manner of EFI, ignition, electric cooling fans, lighting and other sundry items, and the variables increase (as do the points of failure).
So, back to basics. Good and proper grounds are your best friend. Second, is using relays wherever possible/practical.
And with all things, "change one thing at a time and thourlly test".
Last, but not least. Document, document, document.
While you mat think your memory is a faultless checkpoint, you will be surprised at the most simple, obvious thing you overlook because you based your work on your memory, rather than your notes...
But, the upside is that you will never forget the thing you missed, when it laughs in your face.