Super charging my 67' 273 commando valiant

Man, I'm sorry to hear you're hurting physically, and wish you all the best.

Well, notadvice but
Have you researched 10.5Scr and supercharging?
I have never done it, but I know you can only run so much total cylinder pressure with iron heads, and; depending on the cam, 10.5 with a supercharger cam, is gonna already be near the limit. It would be a huge disappointment if after spending your wad,you come to find out you can only run 2 or 3 psi before detonation breaks the pistons, or worse.
I think turbos might be a little easier on her.
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If I was in your situation, I would build my dream system;
I would engineer a two turbo staged system, with very low Scr, and a little turbo up front on the passenger side, and a bigger remote mount at the back. Like you,I would target 400ish hp, because as a streeter,400hp is a lot, already having major traction issues. It's also, a lotta fun.
The little turbo up front would start very early and make the engine torquey like a 360. And let it would let me run a small enough gear to hit 60mph at the top of first gear.Like 2.94s. It would also spool up the remote.
The remote turbo would pump up the secondaries, after the tires hook.
In my dream I mount fake sidepipes, that are actually the cold side from the remote, and no mufflers at all. In my dream the remote pulls air from inside the ventilated trunk. My dream starts with a 318 for a little more NA kick,(and easier parts selection) and it's easier to hit the 400/450 mark which, for a streeter,(ME), is all I could ever wish for, having a NA 360 already in that zone, and being very happy with it.
My dream uses a big spreadbore carb on an old Offy DualPort, with the little turbo jamming the primary side, and the remote stuffing the secondary side. This system uses no intercooler, but the carb would get fresh cold air, and it may use water injection with open-chamber iron heads. But on a 318, I would install aluminums so I can run a lot of Cylinder pressure.
But at Age 65, My dream,sadly, will have to remain a dream.
Happy HotRodding.