I also worked on the Ford assemblyline in Chicago, building the 1977-79 T-Birds so I know these cars like the back of my hand. I've had 4 of that style Birds over the years and they are very nice riding cars with not great but decent gas mileage.
These cars bring in from $1500 up to $10K depending on mileage, lor and condition. Taken to a transmission shop will raise that price to about $450.00 for the labor to R&R it from the car but that's a whole lot cheaper than rebuilding a computer operated overdrive tranny at about $1500-$2000 so your barn find T-Bird is probably a good deal to take even with a bad tranny. The regular 3 speed automatic transmissions are very simple to rebuild and can be done on a bench for about $150.00. The Ford AOD overdrive tranny wasn't available back then. A '79 T-Bird will either have a FMX or a C-4 automatic transmission.
If the price is right, I would buy the car and just have the trans rebuilt. Whether it's got burnt reverse clutches in the trans or just clogged up, the car has obviously been sitting a long time and after that the tranny will need to be gone thru anyway or you might tear up more clutches driving it like that. It sounds like the tranny will need a rebuild either way.