Welcome to 78TA! Be prepared for an avalanche of requests for pictures!
It sounds like whoever did the auto/4 speed conversion wasn't meticulous with the wiring changes. The backup lights are controlled by a reverse-lockout linkage that connects the steering column with the shifter position. Obviously the mechanical linkage is different for a manual than for an automatic, but the switch is located on the base of the column so the idea is the same between the two. Obviously there's been something that's gone awry with the plan.
Your best bet would be to get a copy of how it's SUPPOSED to be in front of you. A factory service manual for your year of car will contain detailed diagrams and explanations of both setups. Once you know how it's supposed to be for a manual setup, then you can compare that to what you have now.
The correct factory service manual isn't hard to come by. You can find printed copies on eBay or other sources, but it's easier to just order a CDROM from Hitman here on the site -- that has the manual on it in PDF format (Adobe Acrobat reader) AND both parts of the parts book for your year of car. Check on the main 78TA page under Restoration Manuals. I have both a printed copy and the CDROM version, and I wouldn't own one of these 30+ year old cars without having that information.
