I think maybe the 4bbl cop-motor also had the larger chamber 360 heads (308 casting?) which caused the lower compression.
What do you guys think of asking for an "economy" forum on Moparts where we could share results on different combos and ideas to save on actual driving costs for our daily drivers? Even with all the OD products available, I don't think many of us are trying to squeeze mpg out of our big block beasts, but more of us are trying to build fun drivers that are on par with newer cars mileage wise. This is new territory for a lot of us hot rodders, and sharing info is good!