are you talking the rear brakes or the front?
For front brakes the Grenada swap was the way to go years ago, but the grenada brakes are smaller than stock mustang brakes of that year.
Unlike earlier mustangs, the drum and disc brake versions in the 71-73 model use the same spindle so getting parts for your car to convert it is easier than getting the grenada parts to swap over. Do some research and you should come up with a suitable option.
If you mean rears . . . SSBC makes a conversion for the 8" small bearing rear end in your car that should be under $650.00