Upgrade to 273 S (Commando) cam?

The factory Commando package included 10.5:1 pistons.
Are they required for the cam? No, but every little bit helps.
Your bigger issue will be changing to a 4 bbl. '65 and earlier 273s had a different manifold bolt size and angle, so manifolds for '66 and later engines are a no-go, at least not without some reworking. Your bolt-on options are pretty much limited to the '65 factory 4 bbl. manifold and the Edelbrock D4B, maybe a couple other oddballs out there. The factory manifold is also only drilled for the narrow early AFB pattern, requiring an adapter plate for a newer carb- but not an issue with the D4B. Another option would be later 273/318 heads, which would open up all the later LA manifold options. Just don't go too new, as the chamber volume increased in later years and will kill what little compression you already have- plus there may be exhaust manifold fitment issues with '73ish and up heads.