Thanks for the comments, everyone! I sincerely appreciate the advice. My slow but (un)steady progress continues. Today I installed my new firewall with a heater “bump out” modification. To back up a bit, after mocking up this potential modification last month, I decided the FFR firewall was too flimsy to support the heater motor. So I purchased a .090” firewall from ffmetal.com. It arrived quickly, and it feels so much sturdier than the FFR firewall – and it’s a perfect fit. Meanwhile, I asked a local sheet metal shop to replicate my 9” x 6.5” x 2.5” box with 1” rivet flanges, and it came back looking really nice. I cut all the appropriate holes and sent everything out for powder coating.
Here’s the box, which oddly looks like some sort of robot emoji:
https://thefactoryfiveforum.com/atta...3&d=1594348739
I’ll call him Mr. Roboto.
I riveted Mr. Roboto to the FFmetal firewall, then I glued my original FFR firewall (trimmed appropriately to sit flush with Mr. Roboto’s flanges) to the back of the FFmetal firewall to create a 0.13” composite firewall:
https://thefactoryfiveforum.com/atta...4&d=1594348758
I went ahead and drilled the three 1 ¼” holes that I’ll need for the wiring harnesses while I was at it. Here’s the new extra-thick firewall with heater bump-out box riveted in place:
https://thefactoryfiveforum.com/atta...5&d=1594348780 https://thefactoryfiveforum.com/atta...6&d=1594348803
You can see the sandwich structure here. This firewall is sturdy!
https://thefactoryfiveforum.com/atta...7&d=1594348842
You can also see the Mike Everson under-dash support panel here, which I’ve still got cleco’d in place. The large round tubes are the heater vents. The small stubs on the right are rivnuts for attaching the dash from underneath. My plan is to have a fully removable dash, so I am looking into the drop-down mounting tabs that others have suggested. I'm OK with with the screw heads showing on the dash -- I just want to make sure none of them are "captured" by the body once mounted.
Next up is wiring!
https://thefactoryfiveforum.com/atta...8&d=1594348859
But I’ve got a 1-week sailing trip starting on Monday, so wiring will probably have to wait until later this month.