Regarding the C-130 firewall issue, I had similar reservations about attaching the firewall to the fiberglass nacelle, without further support.

With my first attempt, I tried a plywood "profile" approach, using a single 7/16" thick profile, with the engine mounted on its side. I epoxied the profile to the wing and fiberglass nacelle. The result was a tremendous amount of engine vibration.

I attempted to solve this by balancing the props and spinners, but still could not get the vibration to an acceptable level. After some research and a lot of head-scratching, I determined that "profile" engine mounts are inherently flawed in that they are simply reacting to this little oscillating "mass" called a piston! With the piston at a "right-angle" to the plywood profile, it can't help, but vibrate - a great deal. In fact, if you were to observe any profile type airplane, you will note that they all vibrate excessively. All this to say, this was not a good idea for the C-130.

My second attempt is now underway and will incorporate a traditional plywood firewall epoxied to the fiberglass nacelle, with the addition of two plywood supports behind the firewall. These two plywood supports will connect the firewall to the wing, augmenting the support from the fiberglass nacelle.