Scary, hopefully it has been machined.
Has "the slotted groove" been machined into the backplate, or has it been formed by adding material on either side of the groove's position? If the latter, can you be sure the extra material won't come off one time you apply the brake?
I imagine aside from the brake plate issue, he will have to use the existing '65 axle as doesn't it have the threaded axle and right side fork leg? Then there is the bearing issue, etc.etc.