I'm at 300 hours on my 700 SCI engines. I'm going to pull the heads and have them rebuilt. What else should be done at this stage? Do lifters and rockers need to be replaced or can those wait until 600 hours to be replaced?

Definitely rockers. Weak link along with springs. I’d do lifters while you’re at it too. Good MLS head gaskets. Have headers pressure tested if you haven’t done that already. Normal maintenance like supercharger oil etc.

All the above plus have your injectors flowed then rebuilt, replace mechanical fuel pump, replace the small fuel filter on the rail, replace your IACs with OEM correct IAC and gasket. Good idea to change your plug wires and test your coils.
Make sure all service bulletins have been performed if your engines qualify (header update, over current protection, etc)