I expect that right now you have a standard angled shower arm coming out from the wall. Grab that with a 12" channel lock and gently try to unscrew it. The one pitfall of this is that if it is old and badly corroded, it could break off. But most of the time, it will unscrew OK. Then you will have to measure and possibly by trial and error determine what length of straight 1/2" IPS brass nipple you need to use to attach that shower fitting flush to the wall.
That is a nice unit. Plan on using some teflon tape or pipe joint compound on the theads of the new nipple.