Quote:
I was wondering if it would be advisable to upgrade to power brakes.
Advisable...? Well, it's certainly possible. There are benefits and drawbacks; only you can decide which outweigh the others.Quote:
Will converting to power brakes make a great deal of difference?
It'll make the pedal easier to press...is that the goal? It will not shorten the car's stopping distances, unless whoever principally drives the car is extremely weak in the legs or knees and has trouble pressing the brake pedal.Quote:
Is converting to power as simple as adding a power booster in between the firewall and the MC, and then connecting to the vacuum supply?
Not quite. You need the booster, the booster brackets, the booster trigger lever, the correct master cylinder for use with a power booster, and you'll need to rebend or remake the master-to-splitter-valve hard lines. Quote:
With a new MC should I get a power booster from a junkyard?
Well, all cars run on used parts! 
This swap is a lot easier if you get all the parts from one donor car rather than trying to piece it all together from multiple donors. The booster brackets from a '71-up A-body should bolt right onto your '73's firewall.