Soft brakes after rear disc install

Your plan will work. I also kept the distribution block in my 1965 drum-drum car since that let me use the existing front tubing. It was just a 4-port "tee" in 1965 drum cars, with single-pot MC with 1/4" tube to the top of the block. I put a 3/16" inv flare adapter at top port for the F tube from dual-pot MC and plugged the rear port. The R tube from MC goes to the existing rear tube via an inv flare coupler. I also installed an adjustable proportioning valve in the rear tubing in case I later change to front discs. Might have been better to have used the one you did to get the pressure-imbalance switch. But, often that switch didn't work in OE systems since got stuck by rust. Since I've used silicone brake fluid in my vehicles I no longer have internal corrosion issues.