My 350 has about 9,000 miles on it since fresh rebuild, and its my daily driver. We got new plugs on there when we first started it up, but I never bothered or wanted to check the plugs if they were running too lean or rich, etc., because I have headers...anyway, I recently put on a Q-jet from a 2-bbl and it really does pretty great. I recently began to think maybe my secondaries were jetted (well, I guess metered is the proper term) just a TAD bit rich because while on the highway then they're open a bit, things seem fine, but if I'm for some reason going WOT on the highway for a while or for some other reason, and slow down and come to a stop, I can smell gas pretty well while still in the car. I didn't think much of it, and again, I didn't check my plugs. I've recently noticed my gas mileage getting worse and worse. I'm wondering if a slightly rich condition can lead to the plugs slowly fouling, maybe make them burn cold (dunno on that part), and my gas mileage to drop like a rock? Thanks in advance.

The plugs will certainly foul if the car is running too rich and that will poytentially affect gas mileage but to get to the point the mileage drops like a rock would most likely have the engine running pretty rough.

You really should get the metering corrected as along with the plug fouling you will also be getting the oil dirty and possible some cylinder wash from the fuel not burnt.