Okay, I did a little more investigating. I emailed AFCO the manufacturer of U.S. Brake oversized calipers. I asked them if they would be appropriate for my 87 GN. Here's what they had to say:
"These oversized calipers would increase your front braking but could cause ill handling. If your front brakes out-stop your rear brakes by too much, they can lock up prior to the rear brakes engaging well and cause handling problems when braking and turning at the same time. For improved braking, I would recommend replacing your rear drums with disc brakes and then look at replacing your fronts with our oversized front calipers."
On another thread, I found that a couple of people were having fitment problems with SSBC calipers, which were my first choice. I think at this point I will go to old reliable NAPA and just purchase replacement calipers and rotors.