Z-bar frame ballstud bracket location/repair help

Heres one for you all that I need some advice on. This is another hacked repair that was done for the Z-bar. It looks like the firewall got rotted out, so someone drilled through the frame to add a mount in order to make the z-bar work. I want to put it back to normal with the proper frame ballstud bracket pictured. The question is, how do I know where to weld it to since the dimples are gone. I do not want to weld it in and then have an alignment issue when I drop the motor and tranny in.