Can someone tell me if the stock LS1 engine has hardened valve seats ? I'm sure they are hardened, but how hard are they, what material is it and what temperatures can they take ?

Why I'm asking, I run the engine on LPG, liquified petroleum gas, quite popular over here in Europe, dunno about the US though.

But the thing is, combustion temperature is slighty higher on LPG, and if the seats are not hardened or hard enough, I might run into trouble.

Where could I get better valve seats if needed ?
Or do they need to be customized for every head, so I need an engine shop ?