Measure twice & cut once. you may not need to use all of the replacement panel depending on how bad rust is. Mark a few reference points on both panels with a texta ie use the seat belt holes, seat mount holes & from the edge. Drill out the pop rivetts at the front, re-use the holes in fiberglass.Overlap panels @ join by about 15mm . Tack with mig & hammer joint to shape, Panel has to be continuos welded on at least one side. Lots of grinding cover all glass with cardboard, inside & out,grinder sparks will embed in glass & will be stuffed