FWIW - the original tires on your '66 were 7.50x14's that were 27" tall.
I don't see any problems with the torque of an M20 equipped 455 and a 2.73 rear gear when you run short 25" tires, based on my long term, personal real world experience
I have been running 3.08's with my M20 equipped .030 over 455 with 26.7" tall rear tires since 2017.
I've bought this car as a project in 1992 and started driving it in 1994. I have run a non posi rear gear with both 26.1" rear tires and 27" tall rear tires with the original 389.. I then swapped in a 3.36 posi rear with the 27". When that rear end went south, I went with a 12 bolt with 3.42 gears with the 27" rear tires. In 2013 I built the .030 over 455 while still running the 3.42's.
After the engine swap, I realized that I had much more rear gear than necessary and swapped in the 3.08 gears with the 27" tall tires. In 2017 I went to 17" rims and now run the 26.7" tall tires.
My favorite car show is over 200 miles one way, with most of it being interstate highway. I still have the same clutch I put in back in 2013 when I went with the big motor, so no "clutch killer" by any means based on effective first gear ratio with taller tires than you are running.
A 25" tall tire makes 803 revolutions per mile, a 26.7" tire makes 257 and a 27" makes 745 or roughly a 10% difference between my current 26.7" and your 25". My 3.08 and your 2.73 are about 10% different, so whether started out in first gear or cruising on the highway at 70 MPH - our results should be very close.
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