[protrash64]

I agree with 61's comments about gas; much cleaner. Be sure to space out the welds so as not to put too much heat in one spot and cause warping. As far as spanning a gap, you have to build it up like filling a hole. A spot on one side and one on the other until you can cross the gap; again, not too much heat!! Grind down the welds and see where you are.

I prefer short strand fibreglass for areas like this as it is extremely strong. It is hard to sand but will resist rust and flex from the bed. Before laying down any type body filler, make sure the work is clean by using a wax and grease remover.