Your information on the hardening is incorrect. Most are hardened in a bath of Nitrate salts. The hardening only happens to the surface .002-.004". Not .020".

This was only done on the Hi-performance stuff.

Any homebuild hardening would be difficult to get a good even hard surface. Any uneven heating/cooling will result in warping. The problem that the factories had. Then the crank has to be straightened. Not a job for a novice.