IMO already contemplating a swap from drum/drum to power disc/drum I would just buy a new stock master cylinder and get the brakes working.  With a tight budget there's no reason to spend the money twice.  Also, going to non-stock parts can cause issues you didn't have before.  IMO its better to get it working and then modify it, so you know what caused the problem.  A single bowl master is all of $40.

The power boosters are the same between 6 and 8 cylinder cars, assuming stock parts.

In a drum/drum system all the car has is a distribution block.  There is no proportioning needed.  A disc/drum car needs a proportioning valve to keep the rear brakes from prematurely locking up and sending you into a skid under heavy braking.  The factory valves were in the rear brake line by the rear axle until, IIRC, the '70 model year when Ford went to a combination valve that combined the distribution block and proportioning valve into one piece which was under the hood. 

Master cylinder bore size is going to depend on the system you are using.  Disc systems use a larger bore because the bore size equates to how much fluid the master moves and a caliper needs more fluid than a wheel cylinder.  Why you typically find power boosters on disc cars is because moving the larger bore master requires more effort.  All things being equal a 1" bore master seems to be what was used in all stock applications for my '67, which had a factory dual circuit design. 

Thanks guys.  I really appreciate the complete and detailed information. I agree TKO, its likely best just to grab a 40 dollar single bowl and see where any other issues might be lurking, then purchase the conversion kit and all the other associated parts to complete the job.
 

Other than the issues with repeated stops, and wet weather driving, drum brakes work just fine. My brother's 69 Mach 1 428scj has 4 wheel manual brakes, and it will stop as quickly as my 13" Cobra 4 wheel disc 69. Go with the new master cylinder and be done.

Qstang wrote:

Thanks guys.  I really appreciate the complete and detailed information. I agree TKO, its likely best just to grab a 40 dollar single bowl and see where any other issues might be lurking, then purchase the conversion kit and all the other associated parts to complete the job.
 

I agree.  my single pot disk/drum mc  lasted 400,000 miles and over 20 years.  It will be easier to plumb too.  Disk mc's are different than drum mc, so be sure to get the correct one.
 

My opinion:  Do it right, one time. 

If you are not going to drive very far, you could use a 70 MC for disc/drum on a drum/drum manual setup, but there might be some slight drag of the front drums.  Maybe that would be a good way to get you off TDC and install the discs at the same time.