anyone know the size of the factory 305 head's valves. thanks

 

1.84 and 1.50

 

if im gonna port the heads, could i reuse the exhuast valves, but install some 1.94 intake valves? thanks

 

Depends on the condition of your exhaust valves.
If none of them have worn stems or are burned or bent.
Take one or two to be reground. Have the machinist regrind
them and then measure the margin. If they are still seviceable
use them othewise get new ones. they are not expensive.
If the margin is too thin the valve will over heat.
The GM intake valves usually only allow 1 valve job till
they are too thin to be serviceable.

An additional 30 deg back cut will help flow.