Chris,
Rocker moldings would have been standard equipment; even on Sprints.
Paint is not original according to your inspection; so pinstripe isn't either. You need to check the back of the quarter panel sheetmetal in order to determine whether the quarter accent trim was there and someone removed it and patched the holes OR the quarter accent trim was never there and there is no evidence of any holes either. If the later is the case, and it's a bon-fide 6 cylinder, then it could have been a Sprint. The odds just got better.
Remember everything on the Sprint package could have been ordered as separate options on any 6-cylinder car. The only exception being the chrome air cleaner cover, which a PO could have simply replaced. The only way to be "absolutely" sure is the build sheet.
If the car you're looking at never had quarter accent trim, given the build date, and the fact that it would be cheaper to order the collective options as part of the Sprint package, I'd say the odds are about +90% that it was originally a Sprint. What you find in way of accent trim evidence will determine that.
I paid $6,500.00 for my Sprint convertible 6 years ago. (But I bought it from a relative) I'd say that a true Sprint was worth $5,500.00. I'd also hazard a guess that the number of Sprints produced for '66 was roughly equivalent to the number of GT's made; which may add to the desirability of this 6-cylinder.