Dave, I'm not sure if you are right that those are ballast resistors. Ballast resistors are used, when the gates of the FETs are at the same voltage (+/- offset) and they are supposed to run almost the same current.
In this case I think they are low side shunts. Also, you have NE5532 per FET, which is a dual opamp, so one is probably amplifying the shunts feedback. This way you can use lower value resistor, which is good because all the reasons Im not gonna list.
Just a minor detail.
Also, I would have liked to see a proper current shunt on the output, probably bolted to the heatsink. You can get shunts for 50A, these coat hangers dont leave a lot of confidence in me. 0.1% max error is not a lot, when we are talking about 50+Amps. I wonder if they are in spec if you run if full load. 50ppm-100ppm/K from these shunts are not unusual, than you have self heating, ageing (can be huge for shunts under load), and issues with layout and terminals. I doubt if they are well within spec.