The '64 Tempest GTO (GTO was an option package, rather than a separate model, that first year) would have possibly been the first "mid-size muscle car", if you don't count the (pretty much the same size) "full-size" Plymouths and Dodges of the '62 model year. With the B/RB motors, of course.
When the '55 Chrysler 300 was conceived/produced, there were nothing but full-size cars . . . even if the lesser nameplates (including Chevrolet and Ford) had shorter wheelbases, but still classified as "full-size" back then.
The lighter-weight Chevrolet would be "quicker", but ran out of rpm due to their "deep" gears to get the quick acceleration. Ford had the supercharged 312 Y-block V-8 in '57, but nothing in '55 that was really powerful. But the C-300 had the top end power to receive speed records on the flying mile at Daytona Beach (on the sand!).
"Muscle car" or "Car with muscle under the hood", the Chrysler 300 would be both!
But then there was the '56 Plymouth Fury 318 2x4bbl that was much closer in performance to the C-300 than anybody in Chrysler cared to admit! But that was in '56 and the C-300 was 1955.
CBODY67