Backfire through Carb?

Finally got my custom build 302 up and running but can't get rid of an idle backfire and backfires on initial acceleration. Have adjusted Holly accelerator pump, idle jet screws, increased accelerator squitter from 31 to 37, squitter engages immediately. Increased power valve from 6 to 9.5 (vacuum = 11 at idle) Adjusted floats. Moved timing all over,(seems to like it around 3 to 4 degrees, but it still backfires. Runs good at higher rpms, no backfire at all. Compression test shows cylinders from 175 to 183. I am starting to suspect crossfire in the distributor, but it's all new. Need help?

Engine is as follows:

91 Ford Roller cam 302 block.
KB flat top pistons (about 10 to 1 compression).
AFR heads 185's with 58cc. (1388's)
Summit Racing stage II vented intake manifold.
Holly 4150 Street Avenger with vacuum advance secondary’s, electric choke.
Holly Blue fuel pump set at 7 psi.
Comp Cams XE264HR-14, roller cam, 110 degrees separation (not too radical) set at about 3 degree advanced.
Mallory SS42 distributor with vacuum advance.
Mallory Hyfire 6AL
Mallory coil 29440
Taylor 409 - 10.4mm plug wires
Champion RC9YC (one step colder than standard).
Moroso electric water pump.
Full length ceramic coated headers, 1 5/8" to 3" collector, 2 1/2" H pipe, and dual 2 1/2" Flowmaster Delta 50's.
No smog equipment, egr, or PVC on engine (will be going in a 64 Falcon).

I refitted plug wires at dist. cap. No change. Plugs: # 1,2,& 6, are black with soot. The others are a bit darker than before, but that's likely because of the richer carb settings. Ran compression check: 175,175,178,183,180,180,180,180,180. Could it be that the intake manifold isn't seating well with the heads? But wouldn't that show on the vacuum readings?

You divide vacuum by 2,power valve should be 5.5 or 6,your Mallory probably came preset at 24 degrees mechanical try running 10-12 degrees instead of 4.The fat power valve you have is over rich.