Well I know I said you "have to" earlier so perhaps I should claify.
And I mean no offense to those that didn't swap out the M/C. You didn't, it works for you, glad to hear it. Same as those who apparently didn't need a prop valve, I'm glad to hear it! 'Cause that prop valve is a million dollar question for me too. I just come from a slightly different school and while I'm willing to take the 'try it and see' and 'less is more' mindset, like with making a conversion and see if a prop valve is needed, I'm just not willing to risk it with the M/C. Then again that is just me. Thanks for listening.
I'm late to the game here, and probably not really "on task," BUT, I can share my experience with a drum to front-disc conversion on a '72 satty.
Let me start by saying that mine was already done when the PO bought the car, so I'm working on a conversion from at *least* two owners ago, undocumented. As best I can tell the calipers were from a '73-up car, along with the spindles. The master was changed to a disc/drum master, but the prop/metering valve under the master cylinder wasn't swapped.
It left me with two problems -- the front brakes not really carrying their load, and the front calipers locking and holding pressure. Knowing that I have a NEW disc/drum master, the line pressure can only be coming from the drum/drum prop/metering valve. I recently acquired a proper disc/drum prop valve, but haven't had time to install it.
As for all the conversions that have "worked," I've read, and seen enough accounts to believe that it works for some without swapping the master and/or valves, BUT, I also suspect many of those are "perfect storm" examples.
As to the question as to whether its "the MC" or "the prop valve" that's important, let's don't lose sight of the fact that it's the SYSTEM that's important. I'm not against experimenting and swapping one part at a time to watch the effects, but for me, I want to get the whole system "right" and the closer to the original engineering specs I can run (granted it's all "cobbled together") the easier it is to diagnose the "not quite right" portions and fix the whole system.
Bottom line: After my experience chasing down an incomplete conversion from 2+ owners ago, go ahead and swap the master and the prop valve...you'll be better off in a whole bunch of ways.