This is another interesting read about Chrysler’s big cars. I’m certainly not on the same level as senior longtime members here on mechanical knowledge or familiarity of Makes & Models but I find it fascinating.
I know of the op’s ‘65 Monaco 426w 4-speed car and of the many other uniquely factory produced vehicles.
Many times I’ve been helped here or learned were to turn for answers.
The op was trying to use just the engine & transmission as the bar for rarest. @PeugFra picked up on that and that is the first order.
Maybe start with that criteria because it seemed the discussion increased the options to include.
Based on that would the op’s Dodge Monaco be the rarest, if the certified documents are available?