I have a 1985 Camaro and started to feel excessive play in the steering. When check on the problem, the inner frame area, by the steering box is cracked, which is allowing the steering box to move when the wheel is turned. Is there a plate to fix the frame or will welding do the job? Is it possible to weld and then weld a plate over it? All help is appreciated.

If you grind a bevel/chamfer along the crack so that welding material can build up and penetrate, then it can be rewelded.
But..
You will need to find where the crack is starting from. Any sharp corner or hole my be the source. If you don't find out why and where it's starting from, it will continue to keep cracking.