Don,
Late to the conversation here. I installed Mag heads on my low mileage '74 360 LA short block (from a van). Dished stock pistons, about .10" (!?!) in the hole. Shaved my Mag heads .020" and got the calculated compression up to 9:1 with the dished pistons. No knocks on 87 octane, has run a best 13.59 in my A body barracuda (no pro tune, I'm sure it has more left). If you installed flat tops without shaving your Mag heads, I'd guess you'd also be in the 9 to 9.5:1 range, which should work well with pump gas and Mag heads. Make sure to get an accurate weight on the truck, and match your cam and torque converter for best results right from the start.