I decided to fix the annoying no-valve-cover-clearance issue on my '78 Cutlass while the motor was out. The heater box sticks out just far enough to make it near impossible to take off the passenger side valve cover, back spark plug, and don't even think about taking out pushrods, when a big block (455) is swapped into it. Swapping to a non-a/c box moves the blower motor into the same place so this was my only choice.
Things I used:
Dremel with a cutoff wheel
12"x18" piece of "Mechanic's Mesh" (1/4" square wire mesh)
Shop Vac (to suck up the fiberglass dust while cutting/drilling)
Resin and giant sheets of fiberglass
First I cut out the old A/C core in the heater box and drilled holes around the perimeter.
Then I formed the mesh and started zip tying.
Then the fiberglass dust that escaped the vacuum got the best of my arm.
I closed up the hole as much as possible with the zipties, keeping the locking part inside the box but eventually the hole got too small.
So I used a couple buttonhead screws I had in the bin.
Then the first layer of fiberglass went on. Pretty simple, cut your giant sheets into manageable pieces, mix your resin, brush a little onto the wire first to keep the sheets in place, then lay the glass, and brush on enough resin to saturate the glass.
I did 3 layers and let it dry for a couple days. It's rock solid and almost as thick as the original box. A quick coat of satin black and she's ready to go.
Then I got a bug up my ass about my engine bay, cleaned it and painted it.