Try changing the filter first.
Additionally, look at the vacuum connection to fuel pressure regulator too, should be located at the front of the motor on the fuel rail.
The elbow on mine was broke, and just smelled stinking rich all the time.
My tank had a definite leak, could fill it, park it for a week, fill it again and it would take 6 gallons.
Thought is was the filler hoses, but it wasn't.
Changed my pump/sending unit because I was chasing a Gremlin (cutting out on every bump in the road), and the gauge didn't work.
Found the Gremlin, wasn't the pump, was the power wire on the plug to the coil.
I broke it off the plug while changing the coil (still chasing the Gremlin) and didn't even know it, so it was already broken, just not all the way. Not a hiccup since.
As for your floor pan, that's really meant to be replaced with the tub removed by a qualified body shop.
The Chief is right, it is a structural integrity issue and I would not use rivets either.
if the rust is that bad, I would personally be looking for another tub, that much rust will be a constant can of worms.
How ever you decide, I sincerely wish you the best of luck.