RE: Hydroplane weight?
I've seen a scale boat that weighed in at a whole 8 pounds INCLUDING FUEL, but I've also seen hulls that weigh over 20. More so than weight, which needs to be minimal, balance is critical. If those two cans are going to be your fuel tanks, move them forward as far as you can so they don't affect the boats balance as they empty. Also, don't hard mount the engine until you have the boat finished except for the decks. At that point you need to have the rudder, skid fin and strut all mounted, as well as your vertical tail mounts. Lay the tails, wing, drive shaft and tuned pipe in their approximate locations and then experiment with engine and radio locations. You want the boat to balance in the 1-1.5" behind the sponson range so that balancing the boat will not take near as much work or weight later