You need to check your spark poor spark will allow plugs to foul.

Quick compression check can't hurt.

Your harmonic is verified at TDC??

was the cam degreed in and where?

I'm really thinking it is poor ignition spark.

Do you have another carb to try??

you could throw the new coil on quick to try. good ballast connections
I had similar problem once with a poor connection.

You could try a double mail spade to jumper/bypass the ballast for a short period to see if it clears up with a hotter spark???

Seems puzzeling but my gut feels spark.