Well, I borrowed this from another forum:
If you look in the pre 63 heads the spark
plug threaded hole has NO machined bevel on the opening, just a drilled and tapped hole. 63-74 have a machined bevel at the mouth of the hole, perfect for the taper of the tube and the washerless
plug. If you look at the drool tube small end, youll probably notice the 45 taper the end was formed with on a 63-74 while the pre 63 head that takes a crush washer will be flat.
It make reference to this:
Slant Six Forum, :: View topic - My 1960 Dart w/ 225 slant 6- what is wrong with this car??
Other than finding the casting number, I am not certain how to tell the 60-62 heads from the 63-up heads.
