here is a link to how a basic vacuum brake booster works
https://www.google.com/url?sa=t&sour...6GYX5O0ctDQ_GD
yours is likely from a corvette or something that had no room for tge booster so they used a dual diaphragm style. anyway, take the pin out of the pushrod to disconnect the rod from the booster from the rest of the assembly, then unbolt the booster from the frame bracket part and youhave the basics for your firewall mounted booster. some will use a direct mount to the firewall at this point, so a large hole for the booster's pushrod and valve body to slip through, with 4 smaller holes surrounding that to bolt the unit to the firewall. it is recommended to reinforce the area of the firewall around that spot to help stop it from flexing, which causes cracks. most "hung" or hanging pedal assemblies will bolt up to the same bolts that hold the booster onto the firewall and this will help with that and also keep the geometry/dimensions between the parts ike they are meant to be. the other end of the pedal assembly, closest to the driver, is usually attached to the rear part of the dash, where the steering column drop bracket is bolted down. it kinda goes without saying that since the firewall and dash were not made to support the load that a brake booster and pedal assembly can place on the relatively flat firewall it is a good idea to build or purchase a kit that is going to spread that load over a larger area. imagine sitting in the seat and pushing as hard as you can on the firewall, simulating the force you would apply if doing a panic stop.
here is a link to a pretty standard set up for a 55-59
https://www.performanceonline.com/19...e-Booster-Kit/
on my 57 I am using the pedals, booster and master cyl from a 2004 gmc envoy. the system has a big bubble shaped formed sheet metal bracket that bolts onto a large hole in the firewall and this is what the entire brake pedal, accel pedal, booster and master cyl system is bolted to. since the sheet metal part is a bubble shaped part it really helps strengthen the firewall there. the steel bracket that holds the brake pedal (they only came with auto trans) also holds the accel pedal for drive by wire. the "inside the truck" pedal bracketry in the stock formation is actually a couple of parts that would be really easy to modify to make fit into a lot of different projects. one wouldn't really need to use the bubble shaped sheet metal part if you don't like to cut a big hole in your firewall to accomodate it and the extra space you would lose under hood from the bubble. the steering column also feeds down through a bellows in the bubble. I will see if I can find a pic