[Mr70]

"So, If my engine stamp is dec 67, I could have a jan or feb alternator or starter if my car was assembled in march/april, but I would need a november distributor."
You could have a distributor on your engine dated up to,but not after the date of your engines running assembly date,which is Dec.67,to be considered correct.
I wouldn't go earlier then 3-4 months max though.*


"So my basic engine castings would be pre-engine stamping date, and accessories could be before or after engine build date?"
You got it,and none of these should exceed your vehicles cowl/trim tag entire assembly date on the firewall.*


*There has been a few documented exceptions to this.