4700 w/a Berk A is a bit low for a mild 460 BBF but not unreasonable with logs. For that power and 61 mph on an 18' jet I'd say a bit slow. But not unlikely.
Do you have trim? My guess's at that speed, shoe and rideplate may be no big deal. Also, your gps 68.2 and 61.2 mph don't jive...unless way differnent conditions. Don't forget river current good for as much as 3 mph if GPS's instead of pitot, and weather variables add to the uncertainty. To measure small changes like shoe or ride plate at that speed you need to do back-to-back testing with accurate measurement.
All that said, to reach your goal of 70 mph with your hull...u may need more power.
jer