"Also in neutral , I can rev up to 5k, not under load. Maybe I shouldn't be doing that"

Yea..don't do that ...'cause you are probably pushing 6000 with no load!!! Alpha water pump impellers don't like that kind of stuff...among other things.

A mechanical tach will work but you have to have a way to drive it along with a cable...not good with a boat. Go to Auto Zone or somewhere and get a cheap electronic tach. Also check for that resistor wire. If I were doing it ...I would run 20' of primary 16 ga. wire temporarily from the coil up to the back of the tach and check it that way.

Running right at 60 with 23" cleavers is telling me that you are "spot on " with tune on your rig.