Welding Tips and Tricks

I put a 3/8 Mild Steel floor in an old steel garden dump cart I've had a long while. Was able to lay the new metal on top of the old rusted floor and welded it in on all three sides of the cart. . On the underside of the cart I had to weld on an axle support to the new bottom ( old floor has now been removed ) I put in. The dimensions of the floor are about 42 inch's length and 28 width. What happened after I welded the support on( this was fully across the cart about in the middle of the floor ) was one corner at the back of the cart ( which is the open end for dumping ) warped so badly that it raised that side of the cart up about an inch an a quarter. I've cut the floor loose from that side with a cutoff wheel. So I have an area of the floor about a foot around from the corner that is warped. I would like to salvage the floor if I can. I'm sure I can draw the floor down to the side again and reweld but if the stress isn't taken out I'm sure it's going to pull that side of the cart back up. I do have a set of oxy accetelene tanks with a heat tip. But have no idea about how to destress the mild steel so I can end up with a flat loor and let el sides again. A y help would be appreciated. Thanks.

Preheat helps post weld shrinkage, sometimes using the peen end of the hammer works(but leaves little divots). Short small welds while backtracking helps me out plenty on sheet metal.