I'm shooting from a box of 2 1/4 x 3 1/4 sheet PanX that expired in 1963. I have no idea how it was stored, but it's providing wonderful results when shot at box speed (which is ISO 64, according to the sheet in the box). The grain is more pronounced than in "fresh" PanX from the late 80's that I've used, but it's still quite fine and there's surprisingly little base fog. I develop it in HC110, dilution B, 7 minutes @65F.
Your film has 15 years on mine and storage matters, of course. They've also seemingly changed the emulsion over the years – I've seen PanX boxes with ISO 32, ISO 64 and values in between. But based upon my experience, I'd recommend not over-compensating out of the gate. Your plan to shoot at 1 stop over sounds like a good starting place to me.