SBP to LBP conversion

Wait what? a 400hp 273? Whoa.....
I'm guessing that will not be Normally Aspirated.
But if it was, that would take a lotta rpm so not much torque involved. And sooner or later you will be looking at putting some decent tires under that notchie, which will lead to a narrowed rear end. So IMO, you might as well start there.
But if not and this is gonna be a street car, I see no reason for discs on the back. The 10x2s are already too powerful for 275s, and you will have to reduce the line-pressure.
I have a 68FB with a 367HO in it, 430hp by the trap in the 1/8th, and after I put the 295s under it, I gutted the P-valve, and gave her full pressure at the back on 7/8 "wc's IIRC, and 10x2 drums. I used the KH'ers on the front with a 15/16 M/C and a booster from a 73 Dart IIRC. Also some braided front lines. After some testing I found this set up pretty awesome for my streeter. It had no trouble hauling me down from 93 in the Eighth, to ~30, for the pit road.
The thing is, my 367 makes plenty of vacuum,with just a 230* cam, and is a clutch-car; so it easily runs the booster.
I suspect a 400hp NA 273 will not have much vacuum, so you might as well leave the booster on the bench;...... unless your notchie too ,will have a clutch, with like 4.88s or something,lol.

Contrary to literature of the day, you can machine the rotors with modern equipment.and
You can pump the pistons out of the calipers little by little on the car, or with a grease-gun, using C-clamps to prevent the movers from popping out early.
I installed silicon brake fluid in 1999, and my calipers are still working just fine.
Yes; I run the Sbp on the front, and the Bbp on the back, Big N Littles, just like back in the day; and no spare. In 20 years I have had one tire go down on me, just 3 miles from home. Unfortunately it was the only time my wife was ever with me. I stuck out my thumb, and within minutes we were home. Then I grabbed an air tank and off I went. For me, not having a spare is no big deal. A flat tire is just an opportunity to meet a good Samaritan.