Scope mount & lower question for the PTR-91 A3S?

I just ordered one of the PTR-91 A3S from AF and want to scope it. What are the lowest mounts I get use on the factory welded rail and still have the scope clear the rear sight? BTW, I prefer twp piece.

Also, I like the looks of the metal lower, is there any reason to not to switch out the factory lower with an RTG steel lower that is clipped and pinned?


TIA