First, welcome!!! Yes, Cougars are sweet cars. Not everyone gets them, but once you’re smitten by their allure, there is no turning back.
So I’m going to wade into this with the disclaimer that this is but one Cougarholic’s opinion. Also, please know that I REALLY like all '67-70 Cougars, which is what you asked about. Knowing that beauty is in the eye of the beholder, here we go:
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Years in order of preference: 68, 70, 67, 69.
Why? I love the 67-68 body style bc Dad brought home a brand new '67 for Mom and we all freaked out. But I agree with Don, '68s are just better turned out cars.
Why a 70 over a 69? I was never fond of the 69 grill. 70 went back to vertical grill bars, and even though the hood will decapitate you, the front end looks better. And yes, the hood will decapitate you.
Trim: XR-7. Love the leather and full instrumentation. The 7th grade me was mesmerized by the tach in Mom’s car.
If you want a convertible you’ll have to get a 69 or 70. There is hardly a more beautiful car than a 69 or 70 Cougar convertible with the top down. 70s are slightly longer than 69s so the lines are “mo betta.”
Engines:
428 or 427 - You will not find one in excellent condition for $30K.
390 - offered all 4 years - stock they’re heavy and sleepy, but can be woken up.
351 - offered in 69 and 70 - 2V and 4V: Generally great - nice power to weight balance - prefer 4V, of course. Debate rages Cleveland vs Windsor.
BOSS 302 - 69 and 70 Eliminator only. Rare, pricey, high strung. Need higher octane fuel than is generally available and minimum 3:91 gears to get it launched. And then it’ll drive you nuts on a long trip. BUT…SOOOO cool!!! And not for $30K.
302 - Standard in 68 - a few 68s had 289s. Prefer 302 4V.
289 - Standard in 67. Prefer 4V.
Transmission:
4-speed, baby! Rare, nothing like 'em! Wish I had one!
Good luck with the hunt! Keep us up to date.