I converted a 68 BCuda from 4 piston KH dsisc to 73-76 2 piston disc setup. Everything else on the car is the same (orig rear and orig prop valve)..Orig master is shot. Should I replace with 68 master cyl or
73-75 master cyl? What is the difference? If it is bore diameter, what effect does this have on braking?

The '73-up masters are more easily and inexpensively available. 15/16" is the bore size used with power, 1-1/32" with nonpower. (Seems backwards, but that's how they did it!) If you have $40 or $50 available, you're much better off with a new than with a "remanufactured" master. www.rockauto.com has good prices.

A smaller bore increases pedal stroke length but decreases effort. A larger bore decreases pedal stroke length but increases effort.

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The most important thing you do is to make sure you get a disc brake master cylinder.A disc brake master will have a smaller front fluid holder than the back one.The front portion actually is the back brakes,and the back portion is for the front disc's.They make the disc portion of the master cyl bigger to accommodate for pad wear.

You can use a master cyl off of almost any mopar,I would suggest one off a 70-72 a-body with disc's.

You can also use a later model alum master cyl that went on trucks and vans.You will need a adapter plate for this particular master cyl.

Bore diameter varies.
whether you like a hard or soft pedal,the choice will be up to you.
Dan or someone else will explain the bore differences for you.

No such master for this application; this is actually three different masters.

'70 power/nonpower: 1" bore
'71-'75 power: 15/16" bore
'71-'75 nonpower: 1-1/32" bore

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一期一会
Too many people who were born on third base actually believe they've hit a triple.