There is a waiting period on the Ford stock X-Plan. I think it's a few months. And it's only good for whoever is listed on the account.
The cheapest way to get a Mustang is to work for Ford (or have a close relative that does or is retired) that can get you A/Z plan. Or work for a dealer to...
I'm not sure if you can beat A/Z plan (3% UNDER invoice) and include all applicable invoice. X-Plan is great if you don't like to negotiate but it's over invoice.
That could be - I found this thread on Reddit
https://www.reddit.com/r/Mustang/comments/7w55ds/important_psa_magneride_backordered_until_2019/
Says Magnaride has been all sold out for 2018.
I'm hoping mine will be built sooner as it has none of those constraints listed in the picture.