You could get 100 answers and all a little different. What I did:
My body had the holes for wipers cut at the factory. I test fit my body and marked on the firewall were the gear boxes would go. I also measured forward from the dash hoop to the center of the holes to see how far forward the gear boxes would fit. I had previously moved my firewall forward about 2".
Took the body off.
Fit the wiper motor, gear boxes, tubing etc using the previous measurments.
Put the body on.
Checked all the angles. Mounted the windshield and fit the wipers on the gear boxes. Checked again. When I knew that everything was going to fit and go together I took the body off again and started working on the wiring.
Same as Mustang Man. A couple of pointers: When you mount the wiper motor, keep the output shaft as horizontal as you can. Even though it is a flexible shaft, it's best to keep it flatter than vertical. Second, the wiper boxes come with short rubber tubes cut at an angle to mount the boxes on the underside. I and several others replaced the rubber tubes with rigid PVC of the same size and dimension to make the wiper box install more solid.
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