You mention detonation at higher engine speed - does it rattle when the carbs open then subside or does it continue to rattle until you tip out of the throttle ?
It would rattle badly until you let off the throttle. Note, at 10 degrees initial, it is no longer detonating.
Did you try disconnecting the vac can on the distributor before you rolled the timing back ?
What is the distributor you are currently running?
Me ?, I'd change the head gaskets as I believe that quench/squish works based on my limited experience... but you first need to figure out what's going on and right now it's pointing to the distributor setup.
