Hi over there. I have a 408 Magnum with 10.2 to 1, Edelbrock ported aluminum close chamber heads and a .555"/540" lift intake/ exhaust, 246*/252* @ .050 with a 108 lobe separation angle.
It cranks to 200psi when cranking compression checked, never pings or runs- when shut down, all while using 91(R+M/2) octane California fuel. It is remapped factory EFI'd and runs very well. I suspect your 99 octane rating is derived using either the Research or the Motor Octane method. Ours over here is the average of the two.
I too suspect the open chamber iron heads. A buddy of mine hade the same issue with a 470' low desk stroker motor. What is your cranking compression?
