I would go with the big power one, I never understood “this has too much power” If it runs on pump gas there is no reason not to. You don’t like the mileage, don’t push on the go pedal so hard. It will help with the highway gears that are probably in the 8 3/4”. There is no reliability issues with a traditional small block stroker and lets all be honest, she is definitely big boned! :):)
Plaza sedan is bottom line 58 (or near it) and I'll bet it's lighter than some 70s A-bodies. Aluminum head 408, and a 904, would be a couple hundred pounds less than an all-iron poly, and iron powerflite.
Edit: Plaza is even down-scale of a savoy in 58. Two door sedan probably the lightest 58 Plymouth you could buy. Frankly, I'm surprised it has a poly and auto, I would have expected a six cylinder flattie and a three on the tree.
