If absolutely nothing was changed (& it sounds like that is the case) but the heads then I'd think that the valley pan gaskets ain't sealing in spite of being changed numerous times. Not intending to bash 440source but they do sell heads as blems on rare occaisions that have porosity problems & maybe one slipped by the inspector(s) but I'm thinking the valley pan. What I'd do is block up the metering rods (all 4) (carter/brock) or a gutted PV's (Holley & both of em) as you said dual fours right?. take her out again & see if being on the power circuit makes it straightens out. You'll have an answer pretty quick