351W sligh misfire and popping after sitting

Hmmm... Well, for starters, your brake lights may just be the switch on the brake pedal. You should have a hot wire going in (hot at all times) and a cold wire which will turn hot when you press the pedal. You won't need the key on as it's on a battery circuit.

I've never used seafoam or any other clensing additive in my engine's oil or fuel. Reason being is, if there IS something large enough to plug up an orfice or what not, that will probably loosen it up and now it's sailing through your oil or fuel system looking to clog up somewhere. Cycling fuel and regular oil changes are the best deal. Putting Premium in the tank is good. I don't know how long you've had the car 'put away' but adding premium should balance out the knarly old fuel. Usually a good idea to put in a fuel preservative into the take when you store a car for long periods of time.

Now for the mis-fire. It could very well be nasty, flat fuel. Try running it down more and cycling more fresh fuel into it. Take a look over your ignition system real quick too. Make sure the cap and rotor all look good with no cracks in such, wires secure, etc, etc.... Could be a sticky lifter, but i tend to think not, especially if the engine is relatively fresh. I don't know how old yours is, so hard to say that. Try adjusting the lifters if you feel that they could be the issue. Sure can't hurt anything and only the cost of a couple valvecover gaskets.

Not really sure what else it could be without being ther by it to see exactly what you're experiencing.

Got to agree on this with Dano, and add.
After sitting all winter, you might want to check for corrosion at the bulb sockets.
Seems that the II's are notorious for wiring corrosion.

sea foam is a great additive probably wont cure your current problem, but is excellent at removing carbon deposits from your valves and combustion chamber,