I had to remove my muffler-back temporarily and ill be putting it back on soon enough, but i needed to do some welding. My question, and it may be a really stupid question, with exhaust exiting under the car instead of out the back cause the gas tank to explode?

 

No? Tons of people have the exhaust dump before the axle. And for the record, it isn't heat or fire that will make gas burn, it is the FUMES that burn. Gas is actually quite a good cooling lubricant (fuel pump gets cooled off by it), and you can actually put cigarettes out in liquid gas. So don't worry about it.

If you are bored and want a quick experiment to show what i mean by gas cooling things... fill a plastic water bottle (one of those plastic ones you get from vending machines), and put a lighter under it... it will not melt while there is liquid in it to disperse the heat. But if you do it while it is empty.. a hole will instantly form in the plastic.