Well replacing the pump while you have the gas tank out if definitely a good idea, but in my opinion it is much much easier to just remove the bed than drop the gas tank. I know this because i did a body lift on mine and the only thing holding your bed on are six bolts, thats it. Then you have acess to everything without removing the gas tank. The fuel pump is right on top.
As far as the exhaust thing goes when your motor is running it generates condensation and this builds up in your exhaust pipes and if only driven for short distances over time will cause the exhaust system to rust. Ive had the exhaust on my truck for several years now and it still looks as good as the day i got it. There are no signs of rust b/c when i drive its more than a few miles down the road which allows time for all of the moisture to escape. Its no big deal, and as far as getting louder having no muffler will make it louder. But in many states having no muffler is illegal, but this is coming from a guy who has neither catalytic converters or mufflers so go figure. Good luck with whatever you do as far as the fuel pump.
Believe it or not, I am taking a trip the night before (1.5 hours each way) and I am going to refuel before the trip and not refuel before the return trip (as usual. I don't let it go below 1/2 tank). With any luck, I should have 1/4-1/3 of a tank by the time I get back. Then I can drive around for an hour with the trans in 3 on the interstate between 2 exits to burn off whatever fuel I can and then I will go back to my apartment, remove anything electronic (computer, GPS, radar detector, radio faceplate, etc.) and the spare tire and then the next morning it will be empty enough for me to take it to the shop and then they can drop the tank with as little difficulty as possible. I was already planning on making sure the tank is as empty as possible before the repair to make it easier for the shop so it makes a nightmare gas tank removal as easy as possible. Is there anything else the shop would need other than tank straps and the pump kit? Maybe I should even bring a 5 gallon jug of gas for them to fill the tank with afterward.
Only one small problem with removing the bed: There is no bed. It's an SUV. A 98 Jimmy. Other than separating the body from the frame, I was told there is no other way to get to the pump. Unfortunately, the designer thought that adding a hatch in the floor made too much sense because it costs more labor to replace the pump after the warranty expires.
My exhaust is rusted already, but not seriously. It was a NY city car before I bought it in Nov 2007 with 58,000 miles. I used to drive alot longer trips, sometimes 3.5 hours each way nonstop every other weekend from the college I went to in Fall 2009 before I transfered earlier this year. This is my do it all truck. Whether it is a 3 mile/5-10 min commute to campus or a 180 mile road trip packed full of computer equipment, it does it all.
As for the no muffler thing, too many people here don't have them. Noone ever gets pulled over for it. I do see alot of American trucks (even some GM S/T series) that are driving around that are newer than mine that have no muffler or a loud exhaust and have entire body panels rusted away and they are still fine. I even saw a 2001+ Sonoma that had a huge rear wheelwell, courtesy of rust. There are alot of loud, lifted trucks here, too. I know better than to park next to a lifted truck on campus because I might find my truck crushed later because some of the trucks have tires that are taller than my hood!