NHRA Welding rules??

And also where I cut the rear frame rail open I opened it up to far and the 2 x 3 tubing does not fit tight, How do I go about closing the gaps maybe 1/8 to 1/4 inch wide? Also cleaned the holes further back in the original frame and welded connector there also

If the gap is too big...take some 1/8 flat plate, cut a piece to fit over the gap ..welding it to the tubing and frame rail.....make a template using cardboard or construction paper...