Big Block Timing with MSD6A and 8495 Billet Dist.

I have a 67 Mustang with a 428 from a 66 T Bird/C6. The engine has about 300 miles on it and I am having some detonation issues. Here are some specs on the motor. TRW 10.5-1 forged pistons, Comp cams roller rockers, Edelbrock perf. RPM intake and Edel 850 Carb, elec choke, FPP Tri-Y headers Jet Hot Coated, MSD6A box, MSD billet dist. I believe 8495 is #, it has black centr. advance stop and light silver springs as that is what the Dyno shop had in stock, (JUST GOT SPRING KIT WITH ALL SPRINGS AND STOP SIZES) The Cam is a Chet Herbert, .544 lift on int. and exhaust, 110 lobe centers. , 108 centerline, Int. duration 289, ex 303, intake open at 14 closes at 40, ex open 53 closes 9. C7ae-H GT heads pocket ported short turn radius worked and hardened seats otherwise stock. Went to dyno shop that had a Mustang Dyno and they said the air fuel mix was rt. but Engine detonates under load. Changed spark plugs to AR33's from Plat AP45's. Also running a 50/50 mix now with 91 oct. and 100Oct. gas. still pinging. Anybody running the MSD box and Billet dist. on a big block? What plugs, timing etc are you using to get rid of the detonation.
The current timing is 12 initial and full advance at 28 per the dyno shop, using the light silver springs. I have the heavy springs now and hope that will help the issue. Any suggestions would be appreciated. THX

Detonation problems

The engine was running between 175 to 180 degrees, actually running pretty cool. The head combustion chamber had been smoothed but not polished. Went to dyno Friday (a Mustang Dyno) and started at 8 initial and 22 total degrees put out 277Hp/347FPT with a bit of detonation. Stepped it back till the detonation stopped wound up at 3 initial/19 total produced 287HP at 4300RPm and 357FPT at 3800rpm. Also changed edelbrock primaries from 6547's to 7047's. A/F ratio 12.2 at idle and 13.3 at WOT. Fuel was 8 gal 100 octane mixed with 7 gal 91. Dyno guy thinks just too much compression.
MSD distributor was running with light silver and blue springs, black advance stop. Timing light numbers were very similar to MSD's chart for that set up. Would the Edelbrock heads lower the compression ratio much over the GT heads?