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Oh, this goes without saying, but if its running too rich, your plugs will indeed foul out. Rods or carb size allowing too much fuel and carburation for the 351 will do it. If you by chance had an Edelbrock 1406 or 1405 carb with the 1456 (073 x 047) rods, you would be all set in the carburator department.
I used an off-road needle and seat in my 750cfm Edelbrock and a Carter Electric Fuel pump with regulated 5lbs pressure. I have found that a lot of 'carb problems' are due to excess fuel pressure blowing open the needle valve and flooding the motor.