i would never seafoam a car. EVER. let alone the gas tank.
my dad tried that once with a car i had--it killed it. dead. it smoked so bad after that i had to have it hauled off.

What i think it did was make the rings stick--bad stuff.
ALSO, depending on how bad it smoked--you could very well have clogged the catalitic converter. they dont like masses of oil dumped down them.
i would say that since the problem started with the seafoam, it's in the things it could effect.
1.) still too much of it in the feul.
1a.) oiled up and clogged the feul filters (an old filter doesnt accomagate oil very well--some people think the gas thins it out, it doesnt--dilutes it, but the oil molecules remain--and stick in the element)
1b.) something broke loose and, if you're flooding out, an injector is now stuck
1c.) could have messed up the o2 sensor (again, oil.. ).
2.) a bunch of crap came off the lower crank case into your oil, OR, from the valves (oiled up carbon, etc) droppped down into the cylinder, and did many horrible things there (for me, it made the rings stick together).
3.) the oil it had to shove through made a already weak catalitic converter plug up.
i dont think the plugs have anything to do with it--they're just a symptom of a larger problem. to change the plugs, pop loose the front motor mounts, that are on top of the engine (dogbones, i've heard them called) and then pull the engine forward... plugs are easy to get to then (pull forward with ropes, a winch, or something creative. takes less than 5 minutes.
those are just my biased opinions.