O’Reilly’s Auto Parts confirms the same master cylinder is used for both manual and power brakes on the 1970 GMC. This is not always the case with different GM vehicles. I’ve encountered Chevelle drum brake master cylinders (for example) that have a shallow or deep socket on the back of the master cylinder where the pedal or booster pushrod engages the piston.
If the incorrect master cylinder is mounted on the booster, depressing the brake pedal will act like an on/off switch and the brakes will lock up immediately (dangerously). This is independent of whether the vehicle has disc or drum brakes, it’s a matter of having the compatible booster/master cylinder combination.
So if all you want to do is add a 1970 power booster to your existing drum master cylinder there won’t be a problem.
The vacuum connection shouldn’t be a problem if you have room behind the carb to add a couple of brass fittings. Simply replace the factory fitting with a T, one port suitable for a 3/8” hard line of hose barb to connect to the booster hose, and the other necked down to suit the modulator.