That's a nice looking job of insulating the mufflers. From my experience it seems to take a very thick insulator to be effective. Mufflers soak up a lot of heat and the large surface area radiates a ton of heat to the surrounding area. Some times it seems a deflector shield is more effective against radiant heat if you have several inches of free air between the heat source and the part you're trying to protect. It's not likely you have that kind of room between the mufflers and the floorboards -- pipes maybe but likely not the mufflers.

Perhaps your insulation will work but if not -- can you fit mufflers in the section of pipe between the rear bulkhead and the axle? Might require a different design muffler but if you can accomplish this it will be easier to effectively insulate or shield the section of muffler replacement pipe under the floorboards. And moving the large heat sink (mufflers) to an open area in front of the rear axle will allow the heat to escape where it won't cause you a problem. Or better yet, move them behind the axle if there is room.

Projects always seems to produce a constant list of problems to solve. Good luck and have fun. Remember, these are just challenges and opportunities.