and if the original seats are serviced-out just use a valve head diameter slightly larger to edge out into new material and start over. FWIW i've heard doing seats in Cleveland heads is not text book and no walk in the park. special shallow seats are required and knowing how deep you can cut is critical.
check out Bill Diehl's comments in this thread,
especially " a lot of research was done to see what kind of seats would work, there is water under there
Exhaust 1750-1450-325 they where custom made be CHE, an off the shelf 1750-1370-375 would need to be cut down "
The increase in flow of Bill Diel's iron Cleveland heads isn't because the inside diameter of the pocket under the valve is reduced with a valve seat insert, it's because it is a five angle cut.
My AFR Windsor heads came that way and as a result with 185cc intake ports flow, get this...297cfm at .550 lift.
My A3 Cleveland heads flow 330cfm @ .600, but that's with a 3 angle cut.
Some of this current technology is Vodoo! It's shocking that it is just this "simple".