On original numbers matching cars, what is the normal time span between the build date on the trim tag vs the dating on the engine and components like the heads and other motor parts? Is it right to assume the body tag should always be after the engine dates?

most things under the hood should be all about one month behind the body built date. but no later then 6 months

My 66 has a 12/2 block date and a 4th week December build. 1965

Is it right to assume the body tag should always be after the engine dates?

For obvious reasons, the build date of the car couldn't be before the casting or assembly date of the motor. You can't build a car without the motor. Keep in mind that engine blocks have two dates. One is the casting date above bellhousing. the orher is the assembly date on the front pad.