As much as I love and use rivnuts for various cover plates and light duty brackets, I wouldn't use them for this application. Even if you use structural quality rivnuts, there is so little supporting sheet metal. I would get some 1/2 inch steel plate and bolt it to the firewall. Then drill and tap mounting holes for the master cylinder mount in the 1/2 plate.
Oh. Just thought of a possible quicker solution. Make the master cylinder mount plate by cutting it out of the front of the booster (assuming op still has the booster and is really ready to trash it...)?
Trimmed the master so the bolts would clear. Also made my rod from pedal to master from an old clutch master rod. Extended 1.25" to a perfect fit.
Blasted and anodized the brake booster delete plate. Zinc plated some bolts for it too. That frankenbolt is the rod from pedal to master. It is now painted black and installed.