clayne gave a good estimate. I found old Tri-X speed was 64 once. So anywhere around 100 to 64 are good speeds to try.

This is why: Since the film is so old it will have very likely "fogged" with age. Fog makes the whole film look a little gray (you would get gray results if you developed it as-is).

So when you use exposure speed 100 to shoot it, instead of the normal speed 400, you overexpose enough to "rise above the fog".

In the end, your negatives will be denser than usual, and a little grainier. You print for longer exposure times, and effectively do not see the fog at all in the print (except for the grain it added).