Other than the 273 heads, only the 302's are closed chamber. 646's are open chamber, small port 318 heads. The 646's might have the redesigned exhaust port as they're the same model years as the 302's, but its not worth a head swap to go open chamber 318 heads to open chamber 318 heads.
The 596's are also open chamber heads, and they're also 360 heads in most configurations, so big ports and even bigger chambers. Without milling they'll kill your compression.
A set of remanufactured Magnum heads would flow better than all of the previously mentioned heads (including the 302's), and the closed chambers will help with your compression. Only issue then is using the magnum intake and converting to the Magnum style rocker set up.