What time span would be correct for my car, maybe June of 66' dated hinges?
The simple answer is yes. Even a May dated set would be OK. The source for hinges was probable close to Dearborn, so supplies would be easily available, unlike San Jose where shipping times need to be factored in.
My understanding is that the hinge finish will be P/O.?
They were natural steel with the springs heat treated. After you clean up many years of wear and tear, the closest finish is a "lite" phosphate treatment for the hinge, darker for the springs.
Be advised that blasting with any media will leave some of the media in the pivot areas, so extra steps are involved.
Jim