The folks providing your plans should be able to give you the center of buoyancy for the hull; this shifts slightly rearward with increasing displacement but usually not too much. You can then calculate moments (weight * distance to center of buoyancy), adding weights ahead of the center and subtracting weights behind. The total indicates the degree of forward or rearward bias in weight. For the side to side trim, of course, it's really easy - just balance about the centerline.
It's just bookkeeping.
I'd definitely locate the engine first; your seating position and the boiler, tanks, fuel, etc. comes next. Passengers get whatever space and weight capacity is left over.
- Bart