73 318 upgrade questions

The best bang for the buck would be putting 1.88 valves in the heads you have,mill them down to get them in the 9:1 comp range. Keep the intake you have and add headers. Gasket match every thing and run a cam in the 268 int duration range with matching valve springs.

Most results seem to show a mild port job will do better than big valves, and with stock compression pistons it really wouldn't be possible to get 9.0 by milling without other modifications like custom pushrods etc.