72 petty blue twister "BACKWARDS BUILD"

This response is pretty late to the party, but in my experience, it does make sense to set the motor, THEN install the headers. I get the motor bolted to the trans with the trans mount bolted in place to keep the motor/trans unit from moving fore and aft. I then use an engine hoist or a floor jack to lift the motor and shift it left or right to slide the pipes in. I hope this helps someone.