I replaced the two standard outside mirrors on my 66 Hardtop with the adjustable LH and matching RH without a template. I used the old eye-ball and opened the vent to make sure they cleared. They were located about three inches further back than the standard ones if I remember. I used a plastic cup washer and screw to fill in the old holes, one on each side, the rear hole was covered by the new mirrors. To relocate the LH mirror further to the rear, you will need to make the control cable hole either an 'oval' shape or make a new one. Take the LH one off and see what you need to do.
Jim