Fibreglass would be there as a reinforcement as well as an adhesive. I was wondering about Sikaflex or Tiger Seal instead of using epoxy, but I'm not convinced they will give as good a bond, and flexibility isn't the aim (the more the dash flexes, the more likely it is to crack).
I spent a few hours on this on both Saturday and Sunday, and I'm basically one step back! The good news is that the new quilted overalls I ordered mean that spending time in the garage is much more pleasant - now I need to make better use of that time!
I started off trying to work out exactly how to make the bracket that joins the top and bottom of the dash and holds the heater vent, and I cut some steel and bent some steel, and found some box that would fit and was generally making some decent progress. I realised I should probably trial fit #5 with the heating vents present, and when I did that, I discovered that it simply wasn't going to work - the bit at the top that was supposed to hold the screw for the cover section clashes with the heater vents.

So I started off making a much simpler piece that would just hold the plastic in the right place and give me a nice sturdy location for everything, so I could make the bit on the other side. I made a simple straight piece with the right sort of bend in the right place, and then I taped that in place and refitted the heater section. And I realised that there was a bit of plastic missing that held all the buttons in the top, and if I put my bracket in the right place, at least I could deal with that. So I cut and drilled and filed and sanded and got another small piece, so there is an L shape, and piece that will sit on top, reinforce it and also bolt up the buttons. (I should have taken a picture at this point, but I was trying to get stuff done.) All that's needed now is to weld them together and trial fit again.
I found my welding helmet, turned on the welder, checked the settings, and discovered the gas bottle was on already. Which was strange, because I try to leave it closed so that there are no leaks, and it's not been used in ages. The regulator seemed to be showing flow (again, strange?) so I tested out the welder, and no gas noise. The regulator didn't change if I turn the dial on it or on the gas bottle, and all the pipes appear to be free from kinks (I can't see behind the welder, but it's not up against the wall). In irritation, I attempted a test weld on something else, but it was horrible and black and splattery and broke far too easily, so I gave up. I think I need to order some new gas.