Which 7/8 master cylinder?

The firewall side should always go to the front brakes........in a disc/drum system. This makes the rear brakes the emergency back-ups

If you hook it up reversed, and the rear brakes go out of adjustment, but not a hydraulic failure, then the pedal will travel lower and lower over time, andno braking will occur in the front system until very late in the pedal travel. If the fronts have any little problem, you may not have enough pedal-travel to affect a good stop.
Whereas hooked up properly, this is an unlikely scenario.
>>If the M/C was not properly bench-bled, and a pocket of air remains in the hydraulic pressure transfer chamber, (the chamber between the pistons), this would cause excessive pedal-travel and "poor braking"..... because the rearmost piston would have to compress that air before achieving some kind of hydraulic loc-up, then the front most piston would have to do it's thing; moving the rear shoes out to the drums, and then braking could begin at both ends.... but that is gonna take a lot of pedal travel. But
If the front brakes are hooked to the frontmost port then three things may happen; 1) you run out of pedal-travel, and 2) the rear brakes will never work, and 3) the front brakes will be spongy, and may/will work "poorly" .
For these reasons, I prefer the tear-most piston to be plumbed to the front brake system...........................but
>>for all this to work correctly, the rear-most piston has to park as close to the firewall as is physically possible. This ensures that the Compensating port is always open when parked, and the chamber is full of fluid. If your pushrod is too long, closing the port, then, over time, the pedal will not return to it's fully up position, and pedal-travel will become excessive, and eventually , you will run out of travel.It will however work fine for the longest time, until the pads become worn enough that you notice the stinking low pedal.And if a minor leak,or a seep,developes while the port is covered, the system cannot replenish the lost fluid, and you won't know about the seep until one day you wonder why your brakes are so poor, and the M/C is still full of fluid!

>> And here's a wild card
If you M/C is equipped with a piston stop for the frontmost piston, and you installed it wrong, driving the bolt into the front piston,instead of behind it then:
The rear brakes cannot work, and
When the rearmost piston hits the front one with the mechanical transfer system, it will stop moving. The pedal will sorta feel like it's supposed to, but perhaps only a small amount of fluid went out to the calipers and so they will be only partially engaged...... and so the Brakes will be "poor".

The same can be said for a M/C that does not have a front piston stop, But the internal stop system has failed siezed, or the fluid is not going out to the rear brakes...... as in somebody put a clamp on the rear flexline,lol..Altho if that was done with the pistons properly parked, then the front system would almost instantly be engaged, as soon as the C-port was covered, with the slightest pedal travel
These are both/all highly unlikely scenarios.
That's all I can think of for now,lol.