This is over on the A/C forum too but here's my latest project.
I wasn't to impressed with the brackets that came with my Vintage Air kit. I couldn't use it anyway seeing as how my firewall is already painted plus I didn't want any hardware showing on the firewall side.
Made this out of 1/2" tubing. I bolt in the upper bracket first, then attached the bottom half of the bracket to the evaporator case and install them assembled. I used Nutserts in the sheet metal of the car. The bottom of the bracket attaches to the little shelf there at the top of the toe board. You can see the Nutserts from the engine compartment but it's much better than right through the firewall. The little brace I made for the center dash support really firmed things up too
It is very sturdy and I probably could have done without that triangulated piece on the upper bracket but it really made the mounting much firmer and it gives me something to attach wiring harnesses and other stuff to later on.