Chevy K3500 454 PO171 and PO174

I have a 2000 obs K3500 crew cab with a 7.4l. The MIL has been on and off since I've had it (Jan 09). It has PO171 and PO174 codes stored. Almost every time the truck is cranked, wheter its been sitting 10 mins. or 10 hrs., it stumbles, pops and backfires under acceleration for 8-10 secs, then clears up and is fine until its cut off again. Refueling seems to make it even worse.There is a very bad gas smell under the hood, and it runs rich. Fuel mileage is not very good. Ever once in awhile, while driving and decelerating, then accelerating again, its acts like its starving for fuel for a sec, then is fine. Fuel pressure is 55 psi with key on, and 49 psi running. The only things I have done so far are cleaned the the MAF and also repleced the temp sender for the gauge (previous owner had installed a manual gauge). Any ideas?

I think your fuel pressure is a touch under spec. Is the fuel pressure bleeding off when the key is truned off, The vortec series of injectors had a ton of issues with the fuel pressure regulator and the nozzles leaking and dripping fuel into the intake, can be a potential cause of fire so if you're smelling fuel get it checked and repaired asap

With 7.4 check the fuel pressure regulator, replaced to many to remember because they leak fuel into the intake. They can also cause fuel pressure to fluctuate.