Chooch, the hydro is not a factor, apart from needing to put the prop valve before it so you don't restrict the hydro (which works fine) acting on the rear drums.
The drum size isn't an issue either,it's that the wheel cylinders are larger and take more fluid volume to move before actually acting on the rear drums.
At the moment, you're using 1/2 your pedal travel to take up the slack (NO this is NOT an shoe adjustment issue, for those playing at home) and actually build up system pressure, as the front and rear operate in tandem (tandem master cylinder).
Restricting the rear line will mean the rears will do their work with slightly more progressive rear brake engagement due to the line restriction, but the rest of the system pressure will build up sooner, thus your pedal will be hard and because the pressure higher, sooner, your front brakes will work harder, sooner - which is the root cause of your issue.
Just fit the adjustable bias valve. I can do the math on this if you really want. Save yourself the time, money and effort.