Exhaust 1.50"
Intake 1.84"
Please note that I did not actually go out and find an 83 caprice 305
to measure the valves on, but these sizes are very common on RPO - LG4 engines from 1978 - 1988 305 passenger.
The only way to be sure is to pull the head off and measure them.
Good luck with your conversion and let me know if I can help you with anything else.
