Keep in mind that the '61 Chrysler designs were on the drawing board by '56 or '57. I think Chrysler Corp painted itself into a corner during Exner's tenure. They were so distinctive from '57-'59. They'd taken some big risks, and Exner's fins had set the industry trend... but where do you go from there? Hard to tone things down. Also, I think there was somewhat of a "design by committee" approach that poisoned the well.
As much as I respect Exner's success in taking Chrysler from frumpy to futuristic, his late-career designs were mostly overwrought. And the "revival" cars (like the Mercer Cobra and the Stutz) were pretty absurd. He was trying to recapture that earlier era but it was like chasing a rainbow.
GM really benefitted from Bill Mitchell's direction after Harley Earl's retirement. Mitchell was twenty years younger and knew how to simplify and modernize. Cadillac was the best example - look at how well the '60 pared back from the '59, and with each passing year thru 1964, Cadillacs were carefully refined.
Neil, I'm with you on the the bubble tops- they were excellent. (The Riviera was excellent, too- that's pure Bill Mitchell.)
I feel like Ford was hit-and-miss in that period. Partly due to the smaller cars in the lineup (Falcon/Comet). The full-size lineup was much stronger.