What Mo said. When we did the frame on my 49 I hired a professional welder. He never welded anything more than 1 inch at a time. That sounds extremely slow but you are welding and positioning constantly so it's weld 1 inch on top at the right front then 1 inch at the bottom left rear, etc. That way you never heat up any one point enough to stress the frame and warp anything.

http://smg.photobucket.com/albums/v518/Blacktailer/1949%20Chevy/