George, The current master and booster kit sold for the 1/2-1 ton truck does not have a large enough bore to handle the bigger brakes.
I wouldn't worry about rear disc. I have had plenty of weight on mine and have had no issue's with stopping.
I would put some serious thought to Grigg's first post. Figure out what you want the truck to do, then build it accordingly. Many people start in on a new project only to go in the wrong direction and end up spending a lot of time and money that they don't need to!
I have 3 big bolts. 1940 GMC , 1942 COE and 1946. All I bought because I like them. One will remain mostly stock except the disc brakes. One will have a modern stuff underneath. Not sure about the other one yet.
Sounds like you have it running well. It wouldn't take much to rebuild wheel cylinders and get everything operating as it should. Finish driving it for the season and figure out the brakes this winter. The disc brake stuff that I make will bolt on to the axle, but the master will take some fabricating. I made the kits based on my 46 and the pedal arrangement is different than your 49... I guess I need to find a later big bolt to buy!
