Without knowing the model truck, or your experience in welding, its hard to know where you are at. If you are a really good welder, then you understand warping, sizing, etc etc. If you are learning to weld at the same time, that's another story. We are happy to help guide you through in any case.
I use a MIG wire feed, make cardboard templates where I can, use butt weld clamps to keep things smooth, and weld no more than a 3/8 inch spot in one place, then move far away to put another weld in, just to keep warping to a minimum. A GREAT Resource for really understanding welding is weldingweb.com. The forum there is specifically for the craft of metal working. Also, grinding can warp the metal too, so take it easy with the Makita!
