Kook, I believe tetraethyl lead was originaly used for octance boost, but as a side benefit it helped transfer heat from the valve head to the seat.Normal cast iron guides have no problem with modern fuel.Without lead, the exhaust seats and valves run hotter causing errosion of the seats, thus the need for seat metal resistant to the errosion.
Usually the seats aren't a problem if the engine is not used too hard. But old valve seats get pounded from years of use, recessing the seat and even peening it over.The recessed valve hurts breathing and decreases spring pressure and changes valve train geometry because the valve stem sticks up higher.When replacing the seats, the valve is restored to original specs.By the way, stainless exhaust valves were used as far back as the 30's ,but it makes sense to replace the valves when doing the head, 235's can use Chevy small block exhaust valves.If you'r doing the head, the 70-100 bucks for new seats is money well spent.
