Jon, are you making the tubs from scratch or using some type of fender tub/pre-existing shape for the tubs?
If you're doing it from scratch, I have a good method that will make things easier for you. Cut the inside profile first. And then cut two thinner strips that are the same contour as the inside profile. Tack these two pieces to the piece that will make up the top of the fender tub. This will allow the fender tub to follow the exact contour of the inside profile. Then trim to fit, remove the ribs, weld in place and your done.
Something like this:
Tim