If it helps....
Typically the 67-72 have easier times finding parts. While more and more 60-66 parts are slowly becoming available, the 67 and on have a greater aftermarket support. Of course, the 60-66 are just cooler
Also, the big difference in the 60-63 and 64-66 is the cab. The earlier had the wrap around windshield (Knee Knockers) and the later models have the standard windshield/cab design. As for the grills, they are pretty much inter-changeable throughout. So if you like a 66 grill for example, it can be put onto any of the 60-66 fairly easily. There are many examples of the early cab design with the later grill/hood combos here on the board. I dont have the link handy but there was a thread a while ago specifically about the different hood/grill combos possible.
In the end it really comes down to what you want. What
I would do is to drive the one you have for now, enjoy it, and while doing so, watch CL, Ebay, and other sources for one of the 60-66 in the condition your looking for and once you find one,
then make your decision if you want to trade up

or not.