I like the '67 Marlin as a one-year-only oddity, but in my eyes, it looks too much like other fastbacks to come from Ford/Mercury. The '65/'66 Marlins are gorgeous vehicles, in my eyes. I have a teal '66 in my garage (major project) with a '65 Ambassador front clip welded on. I'll finish it...eventually...
Not sure that I agree with you on the 250 leading to the 290 through 401 engines. The Gen-2 AMC V8 was a completely different animal, altogether. The 250 co-existed with the 327, and the 287 filled the small displacement needs of the buying public when the 250 was dropped in the early '60s. I've even read some (very old) magazine articles of racers putting the 250 heads on 327 blocks for the increased compression. However, I'm sure there's more to it than I remember, as for one, the 250 ran solid lifters, and the 327 ran hydraulics. (*Interesting side note: Many Mopar engines use the same hydraulic lifter as the 287/327 AMC engines).
I have two aluminum block 196 engines in my garage. One is an average-milage runner, and the other has all new internals and a rebuilt head, just waiting for reassembly. The aluminum blocks were built from '61 through '64, and were only used in the Classic models. They were never available in the Americans, or the Ambassadors. Also, at one point in time, there were three variations of the 196 (195.6) engines available at the same time. There was the flathead, the cast-iron OHV, and the aluminum OHV. Talk about variety!