i`m in the process of moving my master cylinders(clutch and brake) to the firewall on my 47 desoto. i have the pedals/brackets from an 88 ram 50. my question is how much will i need to reinforce the firewall? i am making a 1/8" aluminum plate to fit on the inside of the firewall for the pedal brackets to fit on to. should i also weld another layer of sheet steel to the inside of the firewall?

I installed a MC/hanging pedal assembly from a 1976 Valiant in my 1948 Plymouth, which should be very similar to your Desoto. Attached a large piece of 14 gauge steel to the inside of the firewall where the MC is located. Also made two additional braces that attached to the lip under the windshield that the dashboard is attached to, and the hanging pedal assembly. Seems to be very solid now, but it wasn't until I added the additional braces. With just the 14 gauge sheet metal only, there was some noticeable flexing if I really stood on the brake pedal.