Full foam between deck and hull ?

Water has a density of 64 lb per cubic foot. So you need to figure how much water you need to displace. 500 lb of flotatation is about 8 cubic feet, but a 500 lb (in air) boat might not need 500 lb of flotatation.

I added two part expanding polyurethane foam poured in polyethylene bags in spaces in the hull of my center console and added 2" sheet polystyrene (XPS) cut to conform to the inside of the gunwale/cap to complete the task and to attempt to make the boat float upright. Or at least more strongly resist capsizing.

Some people jam "pool noodles" in the gaps between stringers between the deck and hull to add buoyancy to older hulls and reduce the free volume in the bilge there. I saw video of just this issue with a center console. His bilge was filling, he bailed a lot, then tried to get on plane to get home. The free water in the bilge sloshed aft, the boat stood on end and sank in about 30 seconds. I think he would have preferred floating capsized to sinking THAT quickly.