Many thanks, Sabatini- or maybe Flynn-fan. I’m both.
My mother (who despised the Redford version) thought the only possibility for Jay other than Ladd was Steve McQueen, appealing but ambiguous.
I believe McQueen could have pulled it off. Norman Jewinson did not think McQueen could pull off playing Thomas Crown but he did and did it well so playing Jay Gatsby would have been easy for him to do.