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.
i'm going to guess that you were upping the initial timing beyond the 10 degrees and were probably doing close to, or more, than 40 at WOT. those distributors have a staged centrifugal advance curve. I use the staged thing in both my cars but it's a little tricky, especially with a vacuum advance. that cam you have has a short duration intake lobe and i'm surprised the cranking compression isn't noticeably more than 180psi. could have a wide LSA. if you do a 9 degree plate it won't be simple to keep the staged centrifugal curve and vacuum advance without a short heavy spring. you'll probably have to do a full centrifugal advance without the vacuum can simply because the proper springs aren't available anymore. something I found out when I was doing the 6pak thing is that the "feel good" advance curve actually slowed my car down at the track about 2 tenths due to extra wheel spin. my last outing, some years back, was cut short simply because I forgot I had the "feel good" distributor in the engine.
I also was plagued with oil and water contamination in the chambers that limited total advance to 34 degrees. since then I've found that 37 degrees total, with clean chambers, seemed to work quit well.
