One other question.
Does the hole you drill go all the way through the backrest tube and through the bottom of the rail....or do you just drill through far enough to go through the top of the backrest tube only?
The hole should only go thru to top portion of the tube, not all the way through. The pins are only about 1-1/2" long.
To measure and drill in the exact location to match your used backrests, it would be very tricky, as it would be very difficult to measure EXACT with the curved surfaces, etc. My recommendation would be to forget trying to use the existing holes in the backrest tubes. I would
measure to figure out where the pin would go if you were to try to use the existing hole in the backrest, and drill a new hole about 1/2" away. Mount the backrests, drill the new holes, you will end up drilling though the sleeve, and the top of the tube, only a total of about an 1-1/4". When you drill thru those two layers, stop. You don't want to got all the way into the other side of the sleeve, exposing a hole into the hull.
You will then want to take the backrests out a few inches, and just re-drill the new holes in the backrest tubes just a hair larger. Cannot recall what they recommend for a drill size, but maybe just ream it slightly larger for a loose fit into the tube. You only want the pin fitting a little tight going into the hull.
I also use a short piece of 1/8" cord to tie the ring on the pin back to the tube. Those pins tend to want to wiggle back out as you sail/trailer.
Hope this helps!