Hello Little Pony,
Sorry about the 20 dollar price on the plans. The plans are very detailed with exact measurements and list of parts. But the money doesn't go to me anyway. It goes to keeping my site online and free of cost to AB/TB/DL's as well as bed wetters and people who deal with incontinence.
But for your question about a extra long twin bed, the only part you would have to extend would be the side rails. I had to do the same thing when my doctor ordered me a hospital bed which is longer than a twin. A hospital bed is similar in length to your twin extra long. Almost 7 feet. You would only need about 6 extra dowels for the extra foot in length and make the side rails one foot longer. Not big deal at all. But feel free to go with any design you wish. It's all about having a crib you enjoy. Many have come up with their own design. Some great ones out there.
-Stanley