Are you sure the motor is set back? It's pretty close to the firewall with everything in the correct location. I've only built one big block/auto a-body (the others have had four speeds), but I remember the trans dipstick being real close to the firewall. The reason I ask is that if the trans mount hasn't been relocated it won't mate up if the engine has been moved.
Check out the pics of the bare block in place in my son's Dart. That is the stock location. Scroll down and you'll how low the engine sits compared to the frame rails.
http://www.bigblockdart.com/techpages/dck.shtml
