what’s wrong with your 60, they do require a different driving technique, no good at lugging, wind second out to 45, downshift to 2nd as soon as it slows to 40.
when I had one it was happy at 65, flat out 73 as advertised, kept below 60 should give over 20 mpg, about 18 at 70, 25 mpg cruising at 50.
at first I was afraid to drive like that until I bought a lot of engine parts from old hydroplane racer, he told me that a stock bottom end was good for more than 2 seasons at 7000 rpm, but 1/2 season at 7500.