My car is also a 4 speed, it has front disc and rear drum brakes. I did not know the little plunger between the M/C and the booster is adjustable. I will do what you suggest and lengthen it, at least it won't cost me any more $, just a little time.
I also changed the M/C when I did the build. Not sure what the bore size of the new M/C is, if it is the smaller one, will that make a big difference?