Measure the center line of the car off the differential, not the frame. Use a plumb bob dropped off the center of the axle on each side right up next to the backing plate at EXACTLY the same place on each side of the differential.
Tie a piece of line (a piece of chalk line will work fine) around the diff tube on each side at exactly the same distance from the brake backing plate on each side, as close toward the backing plate as you can get. This line will be larger in diameter than the diff tube. For instance, if the diff tube is 3" in diameter, make your line maybe 5 or 6 inches in diameter. The idea here is to have a piece of line tied around the diff tube that is larger than the diff tube and will hang down around the tube. Now, when you run a piece of line through that hoop, with a plumb bob on the line, the plumb bob will hang down toward the floor at exactly the center of the diff tube. The circle of line around the diff tube will look like the lobe of a cam once the weight of the plumb bob is on it. Make a mark with a Sharpie where the plumb bob touches the floor. You will have two marks on the floor, each indicating the center of the diff tube and exactly the same distance from the backing plates on each side.
Have a buddy hold one end of another line down on the Sharpie mark. It will be helpful if you tie a knot in the line and keep the knot on the Sharpie mark. You will be on the other end of the line, with the Sharpie tied on the line. Holding the Sharpie exactly straight up and down and keeping the line taught, scribe an arc on the floor, approximately at the front wheel, fore to aft. Now, have your buddy move to the other Sharpie mark at the diff and hold the knot on that mark while you swing another arc. Where these two arcs cross is the exact center of the car, as referrenced off the differential. Now have your buddy shorten the line by tying another knot in the line, maybe two feet shorter than it was the first time. Same exercise over again. Your buddy holds the knot on one of the Sharpie marks at the diff while you swing an arc with the other end of the line. Now, your buddy moves to the other side, holds the knot on the other Sharpie mark and you swing another arc at the front. The centers of these arcs will be the centerline of the car and you can snap a line, front to rear, off the centerpoints of the two sets of arcs. Then use a 90 degree square to determine another line, either to the driver's or passenger's side of the car. Lay the square on the centerline at one of the arc centerlines and measure how far you want to move the engine/trans. Make a mark. Now move to the other crosspoint at the center of the car and measure the same distance. Now snap a chalk line fore and aft on those two marks and you have the center of where you want the engine/trans. This method will place the engine/trans exactly perpendicular to the differential.
You can swing as many arcs as you want to. Just keep shortening the line each time and tying a new knot in it for your buddy to hold on the rear Sharpie marks.
Last edited by techinspector1; 10-28-2012 at 03:24 PM.