How much trouble and expense is there to changing a pre war safety to the regular pre 64 safety?

You can swap just the prewar safety lever for a Tilden safety lever, no other parts need to be changed. Tilden safeties show up from time to time on eBay and the other auction sites. The Tilden is similar in appearance to the later safety, it will work with a scope particularly a low mounted one whereas the prewar safety will not.

Does the Tilden also change the safe/fire positions to operate like the later models?

Thanks,

Jack

Yes, the Tilden safety safe/fire position is the same as the later model safety.