You may not need a proportioning valve.

Especially if you're running big and little's.

My 32 has the propo valve cranked almost all the way out so no proportioning is done and more than likely if I removed the valve the brakes would be the same.

If'n I was you, I'd plumb in a place where the propo valve could be easily added and try the car without one initially.

I plan to do that with my 31 on 32 rails roadster which has less disparity between front and rear tires than does the 32 roadster.

Fwiw - tire diameters on both cars:

32 = 30" rear, 24" front.

31 = 28" rear, 25" front.

Same engine/trans/rear axle combo in both cars.
And very close to the same brakes for both.

(462" Buick, T-400 and Ford 9".)

(68-70 Mustang 11" discs front, 68 Merc wagon drum brakes rear on the 32. Camaro 10 1/2" discs front, same drum brakes rear on the 31.)