I like your analytical approach to everything. I like and enjoy your problem solving ability. It seems obvious to me that you have an Engineering background and quite a bit of experience. "Carry on!" 
Indeed, I was an engineer 
I ran a full race, manual brake set up on the street in my '68 GT350.
I was using a 1-1/8" bore master.
The brake pedal was firm but didn't require much travel and there was no issue with leg effort driving it on the street.
The stopping power was really effortless and it seemed like I could not lock up the brakes at all.
The car stopped and it stopped straight, so I would comment that it is possible to use a fully manual brake system with no power assist safely and effortlessly on the street.
It is just a question of having the correct components together.
Those were with Porterfield r4s street pads and there was less pad wear with them.
The only issue, if it was in fact an issue, was those pads do generate more dust on the wheels.
You are re-engineering the system so expect some quirks that you must adjust for.
In the Pantera I would expect any kind of larger rear brake pad/caliper combination to need the pressure to them dialed way down. Where it winds up to your satisfaction is up to you.
I chose an MC of 5/8'' for the front and 11/16'' for the rear to have a pressure on the rear circuit lower than that of the front circuit with a 50/50 adjustment and to have a slightly greater volume of liquid displaced at the rear than at the front since the pistons of the rear calipers are larger than the front ones.
For the moment I have completely ordinary pads at the rear but my rear calipers come from a Renault Mégane RS II which is a very sporty car often used in rallye and on tracks, there is a large choice of pads more efficient.
I have to receive the Porterfield pads tomorrow and I hope that it will finally stop raining so that I can do some testing on a really dry road, which I haven't been able to do yet.