Structural integrity of 4 lb. foam

Babylon Boat Works, Hi. My name is Bryan Finch. I've been watching your boat restoration videos on YouTube for quite sometime. I have just acquired a 1973 Mako 20 that requires basically a full rebuild. I have done a lot of work on various boats, but I have never rebuilt a floor. I saw in your video "15' Answer Foam Removal" you were discussing using 4lb. foam to create structure to support the floor. My question is would that work for my 20' Mako? Any help would be greatly appreciated. Thank you.