Fred, This is a long shot, but does your Oil Filter Bracket pad look like this (1st pic). See the the little hole and it looks like the shadow of the old gasket is right to the edge of the hole, possible leak point. You can plug it with Pipe plug as in Pic 2. Don't ask what the hole is or was used for, no one seems to have a clear answer but it can be found on older blocks, it just goes thru the skirt and dumps back into the pan.
As for your concern with the cross under header tubes, take a piece of the sticky back heat reflective sheet and stick it to the bottom of your pan. Also does 5&6 header tubes drop down close to your filter? If so, take some of the same reflective material and stick it on the back side of your filter. How much does this help I don't know but at least I tried.
Bill