Going of on a hunch.
With the cam change you need more initial timing.
I estimate you need 18-22 initial. The thumper cams have a low center line with a lot of overlap, this gives the thumper sound. ( I call them whipper cams)
You need a curve kit in dist.
You reduce the amount of mechanical advance to increase your initial based on setting the total timing.
The vacuum advance advances the timing based on vacuum, vacuum is controlled b engine load. The vacuum advance needs to be disconnected during these setting, once the setting are archived, hook the advance back up.
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Red line is total timing, the blue is initial.
On the left is 25° degrees mechanical advance, the right is 16°
by reducing the mechanical advance, you raise the initial timing.
Hope this helps