The original factory tire size is 8.55-14. Which is 28 inches talll.
21575r14 and 21570r15 are too small.
( Yes I know many run the 2157514 since there is no other choice)
22570r15 is Ok but either 22575r15 or 23570r15 are at the same outer diameter.
Plymouth C's came with 8.25-14 which a 22570r15 is perfect.
Been in the tire biz for over 20 years and did a ton of olderl car tire conversions.
The original tire is 8.25-14 for a standard Monaco or Polara (non-station wagon or non-disk brakes). That's for all engine sizes (318, 383 and 440). If the car had A/C and a 440, the size was 8.55-14. If it had a trailer-towing package, then the size was 8.55-14 for all engine sizes (even 318).
That info comes straight from the 67 Monaco/Polara service manual. If the car had disk brakes and various other options, the size ranges from 8.15-15 to 8.45-15.
I've never come across a historical document or source for measurements for the old tire sizes such as 8.25-14, only matching tables.
Coker tire's specs for a biased 8.25-14 tire show a diameter as 27.6 inch, and show the equivalent metric size as 225-75-14. When you select a G78 tire, the diameter spec is 27.1 and the equivalent metric size is 215-75-14. Coker shows an 8.15-15 tire has having 27.7 inch diameter, and an 8.55-14 as 28.1 inch. I'm unable to understand their bias-look radial tire sizes.
A theoretical 215-78-14 tire would have a diameter of 27.2 inches, and a 215-80-14 would be 27.5 inches. 215-75-14 is (mathematically) 26.7 inch and if this is accurate in a real tire, then I agree it's at least a half-inch shorter in diameter vs what you'd like to have. A 225-75-14 would be ideal, but there are no tires available in that size.
Next year when I have the Hankooks mounted I'll be in a position to measure the actual diameters (loaded and unloaded) of the 215-75-14 and G78-14 tires.
edit: There apparently are 215-80-14 tires available under the Nankang brand and also "Leao tyre Nova-Force Van" brand. European availability, not sure about North America.