early a master cylinder swap

BTW, with drums front and rear and the old single reservior MC, the F/R proportioning was changed by changing wheel cylinder bores. My '62 had the same sized drums front and rear, and the proportioning was done by having a slightly smaller rear wheel cylinder bore:1" up front and 15/16" in the rear.

In your original post, I looks like you say that you have 1" wheel cylinders front and rear....is that right? (It is hard to tell from the text.) I just looked at the NAPA online site for your car and it shows 1" for the fronts for 9" brakes.... BUT 13/16" for the rears with 9" drums.... I think you had better recheck what you actually have in the rear and recheck applications. If you really have 1" for the rear then those look to be wrong. I can gurantee you the F/R proportioning will be way wrong if they are that far off.

BTW, the same proportioning F/R will be preserved with the 1" bore dual MC versus the 1" single MC. So that part is not an issue; it is just your report on the size of your rear wheel cylinders that is of concern.