I haven't personally done this so take it for what it's worth:
I've always heard that a 300 responds well to a small (500cfm) Holley vacuum secondary with the split headers and a small RV style cam. More people might make a 4 barrel intake, but Offenhauser was the only one I've personally seen.
I would go with the others though - 325,000 is a lot of mileage on any engine (even the 300) - I wouldn't try to make a lot of horsepower with it and I wouldn't throw more gear at the thing. It seems like those 300's had the same acceleration if you were unloaded or had 2000lbs in the bed, but they just keep chugging along.
Good luck and let us know what you end up deciding to do!