1973 360 upgrades

Stock pistons is the hangup- you need to get some compression going. Some thin head gaskets like the Mr. Gasket 1121G s (.028 compressed) will help some.
With that mild RV cam and stock heads, a factory 4 bbl. manifold with carb of your choice (I'm kinda partial to TQs) should be plenty. An Eddy Performer or Weiand Action Plus or Stealth would be good choices, too. They flow plenty good for your heads and cam and can be found used pretty reasonably.
As long as you have decent duals right now, headers would be last on my list- and probably if and when you do a cam swap down the road.