I did some searching and it appears this is a sort of common problem. Most likely a problem in the evap system, either a valve or canister. Valve is servicable, canister is not....new tank if the canister is plugged.

72 RR, Pump gas 440, 452s, 3800 lbs, Corked, ET Radials,. 11.33@117.72. Same car, bone stock 346s, 9.5 comp, baby solid. 12.24@110.