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It would be best if you laid out the letters onto a piece of pasteboard with a horizontal line drawn to set the letters 'on level'. Aftermarket letters may (or may not) have the same pin positions as the originals. But, why take the risk. Lay them out so they look good when placed in holes in that cardboard. Then, use that cardboard as a stencil to set the position for the holes you will drill. That way, you know how they will look when you get done installing them.
P.S. The (vertical) position of the letters should be set so that letter centerline lies over the centerline for the taillight lenses. Below is the rear of my car which has the original factory lettering and holes to match, if that helps any.
My car is a bit strange. The car has 100% 1969 factory paint, has never had a damage, or anything. But I have no lettering in the back, and I can see that there have never been any holes for the lettering.
There is absolutely no way a 1969 car can have a factory paint job but no holes for the chrome letters in back. You can check on the inside surface of the rear panel for presence of existing holes, or extruded body filler protruding from those holes (either of which would tell you that the back panel is probably the original factory panel); or you might find that the inside surface is as plain and flat as the outside surface (which would indicate that the rear panel is a replacement piece that was never drilled for the lettering).
My car is a bit strange. The car has 100% 1969 factory paint, has never had a damage, or anything. But I have no lettering in the back, and I can see that there have never been any holes for the lettering.
How can that be?
I suppose no emblems is a possibility, but very odd, if your car was factory exported to Denmark. Not having those letters and still passing the regular GM quality control is unthinkable.
United Parcel Service ordered a whole fleet of MACK truck/tractors and demanded they have no MACK grille emblems and no bulldog mascot on the hood. Mack caved in to the big order and complied. So anything's possible.