That carb seems ok to me assuming its the vac secondary it'll size itself accordingly
Your sure that timing light is any good with the MSD? Used it before on any msd systems? Some lights cant read multispark correctly
At 20' (for example) timing what exactly happens? Does it just stall out, sputter, or rpm drops?
The rough idle and fouled plugs you describe sounds like a carb or spark issues. At 60' timing what are your rpms at idle? It sounds a little like your using the timing to compensate for bad fuel mixture or even a dead spark on a cylinder.