John, if you have a non air car and the original firewall booster there is another option. I have a non air car and looked at multiple routes including the dewey route. I finally settled on going with the dual 8in just as Dave says the bracket is the issue. I removed the bracket from the original and built a 1 1/2 in spacer between it and the new OID booster/MC. The purpose of the spacer is to get the correct geometry on the linkage. With this setup you use all your current linkage without modification. My combo valve mounts underneath. Just another idea.
NCbird on the Coast of NC
"Dads Bird" for my father