Process normally.

There's probably fog due to age. Overexposure gives higher density for the picture over the fog. That's the idea in overexposing old film. Overdevelopment can also increase fog level which is not wanted. Underdevelopment might be good for fog but it probably decreases picture contrast more, so just process normally, it's the least risky.

I would start exposing at ISO 800 but you can't know before trying. It may work well at 1600 as well. Bracket multiple exposures at 200, 400, 800, 1600.