Most polyurethane pourable foams will create about 60 pounds buoyancy, a little different depending on density, but a block of the foam 3' X 24" X24" will float approx 1400 pounds, of which would about equal the weight of the motor and drive of a big block w/drive assy. My boat weighs 2410 pounds off the trailer.
I have installed enough foam under the floor, bow, and hip pockets fo float the boat, not including the air pocket created up in the bow, should it become swamped, it won't be floating up-right, it would be floating in a bow bow up position, thats good enough for me.