[Chevette]

I would recommend putting the brake booster on the firewall. Brake fluid gets warm enough just from braking, add to that the headers in close to the master cylinder and it gets even hotter. That along with the two items RudeDude mentioned and it's good reason to to firewall mount the system. Also, if you mount the m/c under the floor, it is likely going to be lower that your brake cylinders which will require a residual valve. I used to have my system under the floor and it worked ok, but I prefer it on the firewall where I have it now. RudeDude makes a good point that recessing it booster may have your knee resting on your chin. I hadn't thought of that. I better go out to the shop and check my ergonomics.