Two 8v71 is way too much power for 7 knots. You will be severely under loaded. A single
2-71 or 4-53 would probably be about right.
Detroit Diesels in general need to work hard all the time, built for brute strength. They are two stroke engines and were not designed to be lightly loaded and will not survive well.
The boats you are looking at were probably designed to do greater than 10 knots on a semi plane.
If you are interested in hull speed
travels, look for full displacement
boats or find a semi displacement
with the smallest 4 stroke
diesels you can.
If I were to guess, I would think about 6 to 10 gallons per hour at 7 knots for two 8v71 running. Not efficient at low end.