I purchased some 75 sheets of 4x5 Tri-X that expired in 1981. It has an ISO of 320. The owner said it wasn't refrigerated but it was kept in a somewhat cold place (attic?). I've read that you're supposed to overexpose by a stop every decade from the purchase date (ISO 200 becomes 100, later becomes 50, etc), however I find an ISO of 320 to be a strange number. What is the approximate speed of this film? I am aware that I can just keep shooting at different speeds but this is 4x5 and I'd rather not waste film.
You can do an approximate conversion by rounding 320 to 300. But also it's two and a half decades old. You mentioned adding a stop for every decade after the purchase date. Do you know the purchase date? If it expired in 1981, then it was probably not purchased (read: manufactured) in 1981.
My suggestion would be to cut one sheet into test strips, and take separate photos with test strips (in the same way that you would use test strips to test exposure when developing prints in the darkroom). To make it easier to manage the sheets when loading them into the camera, you can prepare the test strips at the same time as cutting them in the darkroom by sticking them to sheets of normal thin card that has been cut to the 4x5 size.
I am not sure where you live, but in my part of the world (central Europe) an attic would be a terrible place to store film. They do get rather hot in the summer.
For a film stored in cellar (another popular storage place) I would try shooting a sheet at ISO 100 and see. I would not buy nor trust a film known to be stored 40 years in an attic.
TRI-X is a film known for wide exposure latitude, and can handle a lot - but then again 36 years is a lot :) Do not be surprised if you do not nail the exposure at the first try.
With expired film, all bets are off. If it’s recently expired and was stored refrigerated it can be shot at box speed. If it’s long expired and was stored in a garage, it may be useless. There’s a “rule” that says you should add one stop per decade expired, but it should be taken with a grain of salt since there are so many variables at play. Still, it’s a good rule for lack of better information and could serve as the starting point for your tests. I would rate it around 100 or even 50 (little risk in overexposing). By the way, lower dilutions of HC-110 are recommended for developing old and expired films that are likely to exhibit a lot of base fog.
My guess is; the ISO has climbed to 400 or perhaps even higher. As film ages its fog level increases. This is due to the fact that the silver salt crystals accumulate background radiation which exposes them. Time and storage conditions do the same. The film becomes hyposensitized meaning most of the crystal have moved closer to their threshold of exposure. No way to tell if the film is fogged beyond serviceability without testing. Develop one sheet just out of the box and another exposed at 400 ISO etc.