PunkInButte

Tuned up, new fuel filter, all at mechanic's shop. Days later, lots of rain, noticed a check engine light, smelled gasoline at rear, headed to friend's garage to investigate & lost 1/4 tank of fuel within 4-5 blocks. Empty tank according to the gauge, wouldn't fire back up, we towed it there. Based on the tank guard condition & general location of leaked fuel & odor, thought the problem to be a hole in the tank. Friend pulled the tank down (didn't disconnect the battery, didn't disconnect tank from filler neck/gas cap assembly, didn't bleed the system) It was late at night and nothing more was attempted until morning (Not at all wise, now I know). It could be many things. Hit a few large puddles, might've water-logged the evap canister. There is, and has been since I've owned it, a P0455 code on the dash when I turn on the ignition x3, and always a slight exhaust odor. QUESTION: What is the procedure to be sure there's no air or contaminants getting in where they shouldn't be as we put this thing back together so we can test the fuel & evap system?
Side note: I do have a code reader, but haven't plugged it in as of yet. Battery has not been disconnected yet.