I have a stock 3800 SC in my Fiero.

I have another SC (hogged out) with a 3.3 pulley.

I want to do a spark plug change first and I got the recommended Autolites 104.

What should they be gapped at? And, will I be okay running with these plugs before the charger swap?

I don't have any engine issues now.

And YES, I know a 3.4 pulley would be ideal, but I got a deal on this one.

It's not so much a question of would a larger SC pulley be optimal or not. It is will your engine be able to safely run that 3.3 SC pulley you got a deal on. And you aren't going to know that until you scan it while at full throttle to make sure your engine isn't seeing excessive KR. If it is, you will probably need to install a larger SC pulley to bring the boost level down to something your engine can tolerate.

The M90 blower design is very inefficient. And the harder you push them, the more inefficient they become. If you don't have enough other supporting mods to safely run a 3.3" pulley, your engine will probably end up making less power with that pulley after you reduce the timing advance to a level that doesn't produce excessive KR than it would if you ran a larger SC pulley and more timing. Again, it all depends on the mods you have and the octane of fuel you are using.

As far as the spark plugs are concerned, I would run Autolite 103's if I were you (which are 1 step colder than the 104's). You may need to reduce gap on these copper plugs to 0.050" or less (from the stock spec of 0.060") to eliminate high load misfires. If you experience a high load misfire, try reducing the plug gap and see if it goes away. Also keep in mind the cheap copper plugs probably won't last very long in an application such as this. I know I was burning up a set in my Turbo 3800 every 12,000 miles - back when I was still trying to use them. I made the switch to Iridium plugs and never looked back (I ran a set of Denso Iridium plugs for many years until finally replacing them last year with Autolite XP103's).

Again, I can't stress enough the need for you to get scan data (and wideband o2 data would be a good idea as well) to make sure your engine isn't suffering from detonation and is getting enough fuel at full throttle (no leaner than 12.0:1).

-ryan

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More is more. Less is not enough.

Custom GM OBD1 & OBD2 Tuning | Engine Conversions & more | www.gmtuners.com

x2 with everything that Darth has pointed out.
Just wanted to add that with a 3.3" pulley, I do not believe that you can tune for power or efficiency by pushing the supercharger this hard without some supporting mods. . I ran into this problem when I tried to run a 3.4" pulley on my setup. Even with supporting mods there was no way that the engine would run without any KR's so a ZZ Performance intercooler was added and that solved the problem..
If you have a good CAI system, a 3" exhaust system, improved manifolds (or headers) a 180* stat and an intercooler you should be able to make some additional safe horsepower with the smaller pulley.
Several years ago there was a guy in the Fiero club that decided that a 3" pulley was the way to additional horsepower. He was of the belief that the smaller you go on a pulley the bigger the horsepower until one day on a WOT run he blew a piston.

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" THE BLACK PARALYZER" -87GT 3800SC Series III engine, custom ZZP /Frozen Boost Intercooler setup, 3.4" Pulley, Northstar TB, LS1 MAF, 3" Spintech/Hedman Exhaust, Autolite 104's, MSD wires, Custom CAI, 4T65eHD w. custom axles, HP Tuners VCM Suite.
"THE COLUSSUS"
87GT - ALL OUT 3.4L Turbocharged engine, Garrett Hybrid Turbo, MSD ign., modified TH125H
" ON THE LOOSE WITHOUT THE JUICE "

I tread carefully. I seldom go past 3k and honestly I don't need the extra HP. But, I want to try this blower out, and maybe drop to a 3.5 only.