Dunno if this helps, but my 292 head had seats installed for 1.50 exh. valves. Wound up using 1.60 exh. valves, and all they did was recut the existing seats. I would bet that if you're not going with valves that are way over stock size, then they'd be able to cut the seats to fit the valve. I'd hope your machinist would recommend hardened seats anyway, so there would be new seats going in regardless, one can assume? Only your machinist knows for sure.
