As you can see this is what I ended up for setback. It does look kinda interesting - except for the amount of work it took to make the body fit. I chose to go that way for a couple of reasons. The first being that the 'A' chassis is a simple design ladder type and everything would fit. The second was I didn't want any more of a heat affected zone by rewelding though the 'A' chassis is mostly 2x4x.125 rectangular tubing.
Oh yeah - Brookville sent the 3" recess rather then the 5" big block firewall that I ordered