Hot Rods 427 Chevy part throttle pinging

I'm having trouble with part throttle pinging with my fire truck. It has a 69 Chevy 427 with "205" 98cc bathtub heads. I believe that gives it 10-10.5:1 compression. I'm running an Edelbrock performer cam and manifold and Edelbrock 750 CFM carb. I initially set the timing at 10 degrees btdc. I have an MSD distributor with a 6al box. The advance curve is the stock curve from MSD. Total advance 21 degrees at about 4000rpm I think. The truck has 4:11 gears and spins at 3000 at 60MPH. When cruising it pings when I keep the pedal steady. If I accelerate the pinging goes away. Coasting it doesn't ping of course. I've tried backing the timing to 6 degrees and I've richened the cruise and power circuit 2 steps and disconnected the vac advance. Doesn't ping as bad but it's still there and it doesn't perform as well at 6 degrees. I'm running 94 octane fuel too......Any thoughts?....What am I missing?

 

Probably need to pull some mid range mechanical advance out of it. Try to limit it a few more degrees. Maybe add a little stiffer spring on the fly-weights. Bath tub heads are a little ping prone. Sounds like you got the basics covered.

I run mine at 15* initial, 8*-10* mechanical, 10* vacuum in the bus. But I only turn about 3500 rpm tops. 9-1 cr on mid-grade.

 

I'll pull the distributor and try that.....so more advance up front and less in the middle.