Hy Noonan, welcome, just a reminder there are two different 55s, the 55"A" or first series and the 55"B" or second series. If you are talking about upgrading to a dual master cylinder and power booster and you purchase a master cylinder designed for drum brakes there will be no need for additional residual pressure valves. As a matter of fact I was a bit surprised that you were only going to install residual pressure valves to the front brakes. If you anticipate installing disc brakes on the front axle in the future you could purchase a master cylinder designed for disc brakes and install 10 p.s.i. residual pressure in each brake circuit, when you eventually install the disc brakes you can just swap out the 10 p.s.i. residual pressure valve for a 2 p.s.i. residual pressure valve in the front brake circuit, hope that helps.
