Factory 4 speed is in theory a big plus (although if the buyer is upgrading engine, the oem transmission likely will have to be swapped too) but V6 and no A/C are minuses (although adding A/C is not that hard.) You might find somebody that wants it as it is, but more potential buyers will likely view it as a good starting point for a resto-mod project. Your car is somewhat comparable to mine - a 39,000 mile Olds Cutlass Calais survivor with diesel engine.
To give you some insight on value, I paid $6,800 (plus shipping) early in 2021 for my diesel t-top Cutlass Calais, knowing I would gut the drivetrain for an LS swap. It was the model I wanted (79 Calais = bucket seats), a color I liked, with the interior color I wanted (carmine (dark red/burgundy), t-tops, no rust, interior in very good shape and only minor dings and scratches on the body. Your car seems to be in as good, if not better shape body and interior-wise. I would rate your car's defining feature of factory 4 speed as similar to mine's t-tops. While some buyers won't care, and some buyers won't like it all, that feature will make the car substantially more desirable to a certain number of buyers.
You are certainly on one of the best forums to find that pool of buyers that will appreciate your car's condition and features. You could set a high number as a negotiable starting point to see what you can get in the classifieds here. I feel like I got a pretty good price on mine, and it seems that prices in general (at least asking prices) have continued to rise over the last year.
If you just want to get the most out of it that you can, you could try the Bring A Trailer auction site. While there are a lot of exotic cars on there, I have seen some good prices paid for survivor grade 70's and 80's cars. I generally think the clientele on BAT is better than what you get on EBay. I know some hard core car folks that will NOT buy a car through EBay, but will buy through BAT. Another possible selling site is Barn Finds.
Personally, I'd keep it and resto-mod it myself if I were you.