IMO threading and modding the Simplex dropout is perfectly appropriate if you intend to ride your PX 10. (Full disclosure - my 1974 LE has a modded dropout and a Suntour Superbe Pro rear mech).

I realize there are people that love Simplex mechs, but as others have said, those mods do not prevent reversion to Simplex.

Some like Delrin and its inherent smoothness, but I'm not a fan of the way it flexes under load.