Those cracks are pretty severe, and I can see where the problem began. Welding anything to the "C" channel part of a frame creates stress risers that result in flexing and fatigue cracks. Boxing a frame without doing some pretty fancy stress relief shaping of the boxed sections can also lead to problems, especially with a high torque engine. Boxing plates should have a pretty deep elliptical shape to both ends to spread out the stress along a long area fore and aft.
I'd suggest looking for a different frame to start with, as there are probably weak spots that haven't gotten to the point of developing cracks yet. If you're going to go with an IFS front end, consider a Nova instead of a Camaro- - - -the wheel track will probably come out better in relation to the fender width. Good luck!
Jerry
