Looking for stainless replacement valves for 340

It is a very common thing to use the chevy +.100 valves in the sb Mopar.

If you really want Mopar valves, try SI Valves.
They are a good valve at a good price. They make them for about any application.

Home of SI Valves

Edit, they also make the magnum valves that Hughes engines sells, and you can get them cheaper direct from them.