On a flat calm, I can do 74 miles on about 7.5 gallons.

Donal

meaning, if you are making 5 knots in calm water you will take 15 hours for the
trip, burning about 9 US gallons of fuel, for about 6/10ths of a gallon/hour
for about 9-1/2 hp.

That's about right.

Or making 6 knots, taking 12 hours and burning 9 US gallons, using about .75
gallons per hour for about 12 hp.

That's about right.

Or making 7.5 knots (rather fast for your size boat) taking 10 hours, burning 9
US gallons, using about .9 gallons/hour for about 15 hp.

Best bet is that you make the 74 miles in about 12 to 15 hours -- or maybe
longer -- in flat calm water.