Amount of Foam

I never did like math questions...
Good fuel proof foams weigh about 4 lbs/cu.ft. Water weighs about 63 lbs/cu.ft., therefore for each cubic foot of foam you will get about 59 pounds of buoyancy. If you want to be on the safe side, you could just take the entire weight of the boat and add that much foam but that would be overkill because things like the fiberglass, wood, carpet, upholstery, etc. are not very dense so they don't need to be floated very much if at all.

The reference section says the Mustang weighs 1600 lbs. If we assume that the weight of the boat is half fiberglass and half steel, we can use the specific gravity of 1.8 for the fiberglass and a specific gravity of 7.8 for the engine and other metals. 800 lbs of fiberglass at a s.g. of 1.8 give us about 355 lbs that we will need to float. 800 pounds of metal at a s.g. of 7.8 gives us about 697 pounds that we need to float, for a total of 1052 pounds. 1052/59 = 17.8 cu.ft of foam required.

Substitute actual weights and foam density for a more accurate result. Don't forget to add the in-water weight of the passengers cause they'll hopefully still be in the boat helping you paddle to shore.

American Composites... I think...two 5 gal buckets (one was foam, the other catalyst) was about $275 to $300. It was 2 lb density and I had some left over.