I built a 70 Duster with a 440. I used the Schumacher kit and HP exhaust manifolds. There is not a lot of clearance in there. Not sure which headers you are using, but if you could find a set of the Schumacher tri-y headers, that would be a big help. I had to shorten the outer tube on the steering column 2 inches to use the manifolds I had. The passenger side manifold fit but I don't think I had 1/4 of an inch clearance. If you use the torque strap, that should help with the engine moving around. If I were to do it again, I would use a 400. The low deck engines give you a little more wiggle room.