Is an 8 1/4", 2.45:1 diff housing exclusive?
at 65 MPH my /6 is purring at about 2K RPM, according to my vintage dwell/tach sitting on the passenger seat.
That would point to a slightly taller tire, with a rollout of 84 inches or 26.76 tall,
like ~215/75-14, or maybe 225/70-14
If you have a 64 cluster in there, then you have a Commando box, my favorite.
And that would mean that 65 =2780 rpm. If you can climb Ok at 2780, that would point to a 3.40 gear, rounds down to 3.23s or up to 3.55s
65mph with 3.23s and 84's is ~2640rpm, and the starter is a respectable 9.98, up from 7.57, or plus~32%
Do you need 3.23s?
IDK;
2.94s would get you 65=2400 and a starter of 9.08 still plus 20% over what you have. I like that better, but if you have to downshift to climb, then 65=3340 in third gear. That ain't so bad, if you don't have to be in it all day. But if you do, then, it will beat up/heat up, the oil pretty bad.