I am looking at purchasing a Master cylinder from a 79 or newer 210. Will that directly bolt on to my 77 B210, without any modification?

I think not as the mounting bolts differ, also B310 has a bigger bore cylinder. But I'm not sure. You might find the answer here: 1200 tech: Brake Swaps. Search for the words B310 on the tech page

I had read the tech section on brake-swaps earlier. I understand that the booster bolt pattern is not a match to my 77. Can I bolt the brake master cylinder from a 79+ directly to my 77 booster? I understand the bore diameter is different, but I fail to see how that would effect me, other than giving me more hydraulic pressure at the wheels.

Larger bore diameter at the master will make the pedal harder to push. Resulting in less pressure at the wheel cylinder for the same effort. Of course, if you find the booster too boosted, this could be a good thing.