And if you want the actual displacement loaded down for cruising just drive your boat to a truck stop, weigh the boat on the trailer. Go put the boat in the water and weigh the trailer empty. Subtract the trailer from the boat-on-trailer weight.... VIOLA - displacement.
Now just add your crew's body weight plus a safety factor and you have a complete number to work with.
Actually, you don't need nearly that much floatation. Even though your boat may weigh, say, 3000 lbs in air, it will weigh considerably less if fully submerged. The submerged wieight is what you need to compensate for. Of course, if you had only that amount of floatation the boat would have neutral buoyancy. It would not necessarily float at the surface. But every bit of floatation above that amount will keep the boat from sinking. And if you have 3000 lbs of floatation down low on a 3000 lb (in air) boat she should float at her waterline even if flooded.
Everything on the boat, including the hull, keel, bulkheads, etc. has a specific gravity that is either more or less than that of water. The bulkheads, for example, are made of wood and would float if submerged. And, as Sumner stated, a full gas tank is very heavy in air but weighs less than zero in water. It has some positive floatation. An empty tank, depending on its weight, can have even more floatation than the equivalent volume of foam.
To illustrate, life jackets do not provide 200 lbs of floatation to support a 200 lb person because he does not weigh 200 lbs in the water. I believe life jackets are made to have 35-50 lbs of floatation because that is all that is needed. The weight of the boat and its gear WHEN SUBMERGED is the relevant number, not the weight in air. Concrete, for example, loses about half its weight when submerged; that is why it is not usually recommended for mooring anchors.
On the other hand, if you base your floatation amount on the weight of the boat in air you are building in a large safety factor.