As Slantzilla said, the '73-'76 OEM disc brakes has a wider track than the '72-earlier OEM KH disc or all of the OEM drum brakes.
Why buy more expensive custom parts? If you need a replacement, you'll need to get custom again.
Use a '73-'76 9' or 10" drum brake spindle and an off the shelf disc brake kit from Wilwood or AeroSpace. I think you will have a bigger brake selection with the 10" spindle. A '72-down spindle will work but you will need the '72-down upper A arm also (the ball joint size increased in '73).