Charlie, as long as the 8.5 (and tank) don’t bring the waterline up so high that the lake trades places with the air inside your boat, that should be plenty.
As a 15 year old, I built a similar sized MiniMost-based boat that was much lighter yet stronger and started off with a Merc KH7 (about 1/2 the weight of your motor and tank), which at full throttle had only the fin and the anti-cavitation plate in the water. It was scary as heck to manouever. I’d planned to find a Mark 20H and rat it up, but a bad experience convinced me to to stop after throwing a Mark 25 on it and going out on a windy day and catching air under the bow. Seeing the boat fly upside down over my head cured me of overpowering that small craft.Although the skeleton of the boat was strong enough to prevent the transom from detaching, I underestimated how bad the weight distribution was.
Moral of the story is, put safety first and talk to people with experience. As you learn how your boat handles at 15 mph, you could consider upgrading the motor.
