You could consider an upgrade to a set of 11" rotors and GM midsize calipers. Several companies make and sell upgrade kits. Speedway motors
, Brake tech Solutions
and Master Power Brakes all have kits to do that, as well as several others.. The prices start at about 300.00 and go up, depending on whether or not you can stand to allow the wheel hub position to move out.
I just swapped a set of the stock size, 9.25" pinto/mustII rotors for a set of 11" rotors with pre 78 calipers on a guys 40 ford and the additional braking performance was awesome.
In his case he originally had some high performance metallic pads with the small rotors and all they did was heat check the rotors because they were working too hard.
If you have the early pinto, (pre 74) no one makes a kit for those, you must swap the spindles to later ones to get the mounting provisions to mount the bigger brakes.