273 Supercharging?

Just shooting from the hip here: I would be looking at CR's down in the 8.0:1 range for this, maybe less. Wild cams like for NA are certainly not the way to go, but I can't advise you on the right numbers.

Have you looked at flow versus RPM for the charger you are considering, with the drive/reduction combination? Too much of a flow mismatch will make problems either way: excessively rising boost with RPM or no boost with engine RPM.

No need to flow heads or go big valves; that is what the charger is for. Just make the valves seal well and the guides good, good valves, etc.

5-7 psi is mild so I suspect you can get by with cast pistons OK; I would certainly start there. I have raced rallies with hypereutectic cast pistons on a 2.6L Mitsu motor with 14 psi boost (turbo) with a true 7.3:1 CR; you can be on the boost for long periods at times and under all sorts of engines loads and RPM's in rallying so that is a good test, IMO. And I don't think the turbo'd Buick V6's in the 80's used anything but cast pistons. I later went to forged piston with a true 8.2:1 CR and 14 psi boost.

The stock pistons and the one model (L2221 for non-dome) available nowadays for the 273 have higher CR than you should use. So, maybe some work to open the combustion chambers might be in order and thicker head gaskets; you can always go to thinner ones to up the CR a couple of tenths.

Are you using the open chamber heads? With those, it may work out to not be too much work to lower the CR.