Just to set the record straight, the '69 was the only year for the rear quarter scoop and on the Boss models the scoop was deleted. Larry Shinoda, the designer of the Boss didn't like the fake scoop on the car. That's why you don't see the scoops on the Boss models...

A quick breakdown of model years that are similar:

65-66
67-68
69-70
71-73

The 65-70 used the same floor pan and other sub structures, but the car was continually widened/lengthened for better ride and to fit big-blocks starting in '67. The 71-73 was a whole different story with revised sub structure, integral steering, and more.

I've got several HPI Mustang bodies myself, but haven't gotten the '70 Boss yet...

HTH...
Mark