250 HEI Timing, vacuum advance question

[felm0001]

Long time reader first time poster,

I don't know too much about timing and ignition, but I used shifty's thread and installed this HEI from summitracing in my 1967 c10 with the stock 4.1l L6 and the old rochester 1bbl.

the truck starts and idles at 550rpm nice and strong.

Under load I and getting pop, bang, rocks and coffee can style ignition + black smoke. I set the vac can wide open at 9deg, then closed it a quarter turn at the time (that took an hour or two). There is definitely a spot there where ignition gets a bit smoother under load (probably around 7-7.5 deg adv out of the can), but still no joy.

I might have the initial timing at 4deg advance, unless each hash is 2deg and the 4 on the harmonic balancer is really an A... then I would be at 8deg initial advance. I set the timing by ear, then put a light to it.

according to summit the specs for this distributor are:
16 degrees of mechanical advance
and an adjustable 5-9 degrees of vacuum advance

so without the vacuum advance I think I am at(16 + 4 or 16 + 8) 20deg or 24deg total advance

with in the last 1-2 months: plugs are new, wires are new, valves are new, pressure check was around 150 on all cyls.

I feel like this is a timing issue, but if the vacuum advance is only adding advance, and it runs fine without the vacuum advance plugged in (I took her around the block), shouldn't the measly 5-9 deg advance only help underload? is there another issue I am overlooking that could cause this poor ignition?