agree, neck on my arabs have been the last to muscle out. There are many good threads here about dressage riding and how to get a horse fit.
I think a good basic dressage program will help your horses' back to become stronger and hind end to come in. \
Correct riding is the key. Longing and side reins may help but it shouldn't be the sole training. Hills and low jumps/poles are also helpful.
If you have access to a aqua treadmill it would be great but I would make sure there is basic fitness. I know an arab that did that right before nationals.
I know a lot of arab trainers and they tend to longe a ton but that is not teaching the horse to carry and use different muscles.
I don't show halter so make sure you will still get the halter look you desire (unless of corse you are showing sport horse in hand, then they all tend to do dressage and hunter jumper for preparations).