I'm not an SF recruiter so I'm not sure about the regs as far as a private trying to transfer over to the x-ray program but as far as getting ready for selection, a little bit of time and a lot of dedication is all you need.
You phrased selection quite correctly. It's just a gut check. A process to see if you're even worth taking the time to train. Most guys will tell you that selection and the Q course (SFQC) is 90 percent mental but take it from me, if you're not one of the top PT performers in your unit then you're just making it harder on yourself when you go through. I was no PT stud when I went through but if I were to do it again, I would have shown up in better shape.
Anyway, here's a website:www.military.com/army-special-forces/training.html
Just push yourself and present yourself as an asset and not a liability. That's what the instructors are looking for.