Danger, Will Robinson!
Disc brakes require a much higher pressure to operate. Drum brakes self-actuate, meaning that once they touch the drum, the friction pulls the shoes into the drum harder. If you look at drum brake cars, they rarely had boosters. Replacing the drum brakes with disc brakes means you'll have to use more force on the pedal, and it may not be suitable for some drivers. The brake proportioning will also be off. Your rear brakes will be doing even less of the braking.
If you're swapping in GMT800 rear brakes, strongly consider swapping the rest of the system in. Front brakes, master cylinder and booster, and (if viable) the ABS system.
Short of that, find a good brake system calculator and do the math to ensure you have a compatible setup. You'll need to know the size of your master cylinder piston, the size of your caliper pistons, and the size of your wheel cylinder plungers.
Calculator for Automotive and Racing Brake Systems, Master Cylinders, Brake Bias, Piston Area, CG Height, Weight Transfer, Pedal Stroke and Maximum Deceleration
brakepower.com