You don't have to relocate the z-bar, it just makes routing the exhaust easier. If you want to keep the stock location massage the firewall enough to fit a 2.5" pipe between the z-bar and the firewall. The attached pictures are of my sons' cars. The first two are my older son's 383/4sp and the third pic is my younger son's 440/4sp Dart. If you are using a late 60s 440 bell you will want to locate the bar so that a single ball stud can be attached to the bell. It's not so critical with the 383 bell as it has two stud holes.
But, here are the perameters you need to attain or get real close to. The z-bar needs to be perpendicular to the ball studs and the z-bar should be a 1:1 lever with all arms perpendicular to the bar (that doesn't mean that they can't have an offset, but the ataching point needs to be perpendicular to the bar). When you relocate the z-bar rearward, both rods need to be shorter. In this case, you want each rod and arm to be perpendicular at mid-travel. The only other factor is that one end needs to be able to be unbolted and slid out of the way to remove the bar.
BUT, these are guidelines. If, to fit, one arm is a little longer than the other, you'll be fine. Or, if one arm isn't quite perpendicular, you'll also be fine. You just want to get as close to these ideals as you can.
rayer:
