There were many different sizes of those back in the day. Some swing-outs put the tire in the center of the tailgate and some put it on the right side of the tailgate. I've even seen ones where there were double swing-outs with jerry cans on the left side and the spare tire on the right side. You might have to just assemble it and hold it up to the back of the truck and mark the holes. I have one on my '76 but don't have any idea if it's the same size as the one you have. I'd hate to give you the wrong measurements.
Just make sure your backing plates are in place or it'll tear right out of the sheet metal.