The rails use the same bed hook design as the original. The picture is from Rockler.com.
The extension of the frame was a bit more difficult because the headboard was designed to curve around the mattress. The finished dimensions of the opening are 60-1/2" (a little wider than it should be, 3/4" plywood was used under the box spring) x 90" (including the curve on the headboard). The additional area at the head of the bed was made into a curved shelf resting on triangular plywood posts, complete with an electrical outlet. It sits about 4" below the mattress and allows you to lay your arm under your pillow without it falling asleep. We can charge our phones, store books, glasses, etc. It has really turned out to be a great addition.
Full Mattress 54" x 75"
Queen Mattress 60" x 80"
Well, it almost worked...The pins I was hoping would work in broke sending the bed into a slanted position. Rather than repeat the same mistakes, I headed over to Lowes and picked up some Simpson strong ties (four 3" x 3" Bearing plates, and four 3" x 7" tie plates)
I removed the hook portion from the old bed rails by drilling from the outside with a small diameter bit and forcing the retaining pins out. Then, I had to trim the expansion rails to remove the area where the pins were and widen the kerf enough to fit the 1/8" bearing plates and deep enough to snug up the old and new rails.
After that, it was a matter of drilling through the wood and plate (but not the outside face) and securing it will #10 metal screws. After getting five screws in each bearing plate, I stretched the 3" x 7" tie plates across the inside of the rail to stiffen the joints. The change is not visible from the outside, but the increase in strength should keep anything from coming loose in the future.
I had to trim the shelf against the headboard nearly 3", but the whole setup is more snug than it was with my original design. The changes took about 2 hours to complete.