For any bonding to fiberglass I will be using West System Six10. Easy to use, can be used for filling small gaps, and the bond is structural grade. For the floor and inside the firewall I plan on using composite board such as Coosa Bluewater 26, King Starboard, Seaboard, etc...a bit pricey, but no chance of rot or delamination, and lighter than plywood. To reinforce the firewall (and dash), I'm planning on T-Slot extruded aluminum beams...one across the firewall parallel to the bottom of the dash, another across the bottom of the dash, and at least 2 stringers to tie the two together. Also to reinforce the firewall, 3/16" diamond plate aluminum, painted to match the engine and trim color.