Yep, the major differences in the presentation between an Arab and a stock-type horse are the leg placement and that stretched-out neck.
I would teach your Arab to set up the same way as you would a stock-type halter horse. There are a few ways to do it, but I like to visualize a horizontal square under the horse's chin, each corner corresponding to a hoof. Moving the lead to a corner means move that foot. There are several other threads on teaching this, just tailor the placement to your horse's requirements.
That lovely stretched neck is best achieved through a combination of lifting the lead to guide the head up and forward, and catching the horse's attention. When I was showing an Arab in-hand in small local shows, I was instructed to do something along the lines of snapping my fingers, crinkling a candy wrapper, or picking up some ring footing and dropping it to get that pricked-ear stretch. I'm not sure what type of showing you're looking to do or what is allowed in the ring as far as those tactics go - that's just what flies at the little open shows that I do.
An "official" Arab halter is worth having if you're really going to go whole hog into halter classes, but I've also seen Arabs and half-Arabs shown in bridles in hunter or saddle-seat divisions, both snaffles and doubles.