There are firms that will take your old master cylinder, bore it out, install a brass or stainless steel sleeve, redrill the various ports and reassemble with new rubber parts. Its pricey, but combined with using DOT-5 fluid (not compatible with regular brake fluids like DOT-3 or DOT-4) will let your brake system last decades. You could locate such firms with a google search, or look around at
www.hemmings.com. And you should probably do the same with the wheel cylinders, also get all new brake hoses, and do an inch by inch inspection of all of the brake lines - looking for corrosion. There is no system more important on any car than its brakes.
I would also advise keeping the entire vehicle original, since a 49 Plymouth is able to handle most modern traffic cruising and stopping needs, without being 'upgraded' - which some antiquers think of as bastardizing. Anything non-original, is likely to cut into the interest other antiquers would have in buying the car. $$$$
Good Luck... They were neat machines.
P.S. I would not set the intake/exhaust valve clearances to the factory specs. It is likely to make it hard to start when the engine is warm. Add an extra .002" to the clearances. It will make a slight increase in valve noise - but it will start on the first turn of the crankshaft. Also be sure to use a fuel additive to replace the lubing effect of tetraethyl lead - for longer valve life.