O.K., seat of the pants reckoning gear setups:
24 front sprocket, 24 tranny sprocket, nice overall cruising ratios,
24 front, 25 rear, good for over the road cruising that 65-70 speeds you asked about
23 front 24 rear, o.k. overall, little better acceleration.
Belt primary, 25 tranny ratio is between the 23 pri and 24 pri gearing. Good all around gearing, happy 60-65 cruising.
ratios for primary:
1.6:1. 23 front sprocket
1.58:1 belt primary
1.56:1 24 front sprocket
Important to have a good balance job on your engine to enjoy the high speed cruising. Been using 60% with good results.
Calculated engine speeds at 60 MPH:
My suggestion and what I use: Carefully balanced engine, and belt/25 tranny sprocket. I can cruise the interstates at 65, and still have good around town performance. This with a 84 or 88 stroker, which are what my 2 road flatties presently are. The stock motor is ok with this as well.