Super charging my 67' 273 commando valiant

10.5 Scr and say a 262 Supercharger cam is gonna put your NA (Normally Aspirated) pressure up to maybe 180psi. So I can't see this working on pump gas even normally aspirated. By the time you add 10psiG you will be up to 14/1 Scr and that's pretty good .............. for E85 or LPG.
To run pumpgas, I think you will be better at 7.8 . But I still think you won't be able to run 10psiG. Maybe 8.
Either way, off boost it will be doggy.
IIRC it works like this; best guess.
With iron heads about the max pressure you can run on pumpgas is 165.
If that is true; 165 guage is ~190 absolute.
10psiG is ..........................~40 absolute
So subtracting them you get 150 absolute what you can design your engine for.
150ab. is ~120 psi on your compression gauge, about.
Designed for 7.8Scr, your 273 will make about 118 psi depending on the ICA, (your cam's Installed Intake Closing Angle) ( I chose 62* and 500ft above sealevel.
That's my best guess working from memory.
Now if your Intake temp is too high (no hoodscoop, no intercooler) you may get into detonation at 165psi cranking pressure, so I arbitrarily downsized your Supercharger to 80% to allow for that. And that is how I got to 8psiG.
I don't think it's gonna make 400 like that..... but maybe...
Best guess man.

7psi, IIRC can double your power,so 8 would be 114% more and so to get 400, you will need 400/2.14=185 to start with, Normally Aspirated. That looks doable at 7.8 Static Compression Ratio and a low-overlap Supercharger cam. But, off-Super it will be less powerful than a slanty. I don't see headers being particularly helpful, other than being roomy enough to park 400HP worth of spent exhaust, for a couple of seconds atta time.
Since a crank-driven super needs rpm to get to work, the bottom end will be doggy, until you install a higher than stock stall, and/or, more rear gear, or a combination.

Keep researching cuz I might be really really wrong, cuz I'm going by an old man's memory.

OOPs I see some math errors hang on. Ok fixed it