Please Help!!! having 87 pickup problems

[bigdaddycaddy39]

I have an 87 chevy 3/4 ton 305/th400 fuel injected pickup 2wd and I am about to pull my hair out. The trans pump went out and the truck sat for a year or so and I finally repaired it. The pickup ran great before it was parked but when i got the trans back in and tried to start the truck the fuel pump had gone bad so i replaced it and started it up and the engine ran terrible. I put fresh gas in, changed the plugs, new battery, new dist. cap and rotor button(old one was burnt up), rebuilt the injectors(leaking after the engine was shut off) computer was throwing a few codes so i replaced the manifold coolant temp sensor, o2 sensor and map sensor. With all of this done the engine is finally running smooth at idle but sputters in drive at low speeds until you build up a little speed then runs ok with just a slight surge at highway speed. The truck never smoked before but now it puffs smoke at idle and blows out quite a bit when you rev it up..bluish grey color...not black and if you mat the gas in drive the truck stumbles..backfires...and dies. the only code the computer has is a high voltage o2 sensor code 43 i believe.I just discovered the rubber seal on top of the sending unit/gas tank is leaking. Can the tank be losing fuel pressure and maybe causing the computer to over compensate fuel and maybe flooding the truck? Any help would be greatly apreciated.