1966 c10 spark knock

[codymwaits]

I have a 1966 c10 inline 6 250. I just did the hei conversion, when my old distributer vacuum advance went out, I did everything right and installed it, set the timing. It idles perfect and revs up but if you hold the throttle down at any rpm like as if you were driving it will sound like gun shots going off. But won't do it at idle or reving. I've taken the valve cover off checked the valves and valve springs, then checked the rocker Arms everything works and look brand new. The motor has 40,000 miles. I need help! Any other things tht could be going wrong other than what I've checked
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