What changed between 67-72 and 73-78 models?

[MVHandy]

Maybe a little basic question but I am going to shop for a truck in the next few months. Style and looks wise I like the 69-78 models grill, front end and overall appearance best.
What I am wondering is what the main differences and changes were from the 67/72 generation and the 73/87 generation besides new smog controls starting in 73. It seems that many of the features I would look for or retro-fit (PS/PDB/AC/etc.) are more readily available on the later generation and pricing maybe a little cheaper.
Can the board members share their thoughts help me resolve whether I need to focus on the earlier gen exclusively or am I okay with say 73-78 as well. Does anyone know thedeal on emissions testing if I have a non-original engine? I live in New Jersey so I'm sure whatever the answer is it will not be simple.

Thanks,
Mark