That combo will probably need at least 20* initial. Maybe more. I would start at 20 and limit mechanical advance to around 16* all in by 2500 RPM. It may want even more initial, if so, you will need to limit mechanical accordingly.
I run 20* initial in my HP 383 with zero or + deck height. Limited to 15* mechanical, with 906 heads with that cam advanced at 4*. That cam would be a dog in the 400 with 8.5. CR. I would either change my choice of cams or rethink my piston combo! I have enjoyed that grind over the years but going back to the RR grind to get get closer to stock.
