BB tune up specs help, spark knock

[veltex]

Ok, I need some fresh ideas to kick start my brain, I am having some spark knock at mid rpms, my info: 396 unsure of comp ratio (saw piston tops a while back though, sort of a pop up but flattened off with just a cut for intake), head guy said the chambers looked like 122s? truck heads for sure though, very even but average compression 130 + or - only one or two. New cam, lifters, intake, carb, guides, springs, seals, been running pretty darn good lately, the package works, uses a little oil, and a lot of gas. Most recent change is HEI, that is when the mid knock was noticed, the changes were Petronix Hei flame thrower distributer, wires, and a regap of 45, thought it was fuel (still could be) been trying better gas, Chevron super around here is only 91, and in winter I think has stuff in it. OK, so yesterday I figure adjustable vacuum advance is to much, back it down a little at a time, not much of a change, dissconnect it totally and put a plug on, makes knock better but doesnt eliminate, truck still runs great without it. I road test timing from 10 to 4 advanced, 8 is where it is now, backed down the vacuum advance from 16 to 8, only seem to get about 16 out of the dist with the middle springs, total is around 30 maybe a little more, isnt 30 total a little low for a mild BB? I dont rev this old girl way up, dont have a tack but would be surprised if when I stomp on it, it makes it above 4k. I am running Autolite platinums at 45, have a new set of AC 45xls, will try just in case thats it, dont hear it at idle or wot, just mid rpm, in any gear if you hold it 1st ,2nd or in drive at about 50 to 65. Very happy with the performance so far of the new dist, it advances great and makes the truck feel torquee, just cant deal with this noise and eventually damage. Even though the total advance # seems low, if I didnt know the #s and the spark noise wasnt there, I wouldnt touch a thing. Sorry for the long post, any ideas appreciated.