The position of the "eye" at each end of the spring relative to the leaf itself, influences ride height. When you choose aftermarket springs for an early Mustang, you have a choice of high, low or mid eye.
Modding your XY wagon springs could be helped by possibly shorter sedan shackles (wagon shackles were often ½" longer), and relocating the "eyes" to below the leaf. This is not your local Pedders/Midas franchise type of work; it requires a proper spring works to do this, combined with resetting.
These pics might help (need to look closely at the differences):
http://www.mustangsplus.com/Merchant...ory_Code=leafs
It's well possible to tie your brain in knots over leaf spring setups. Expect to have 2-3 cracks before it's the way you want it. As to the rear sway bar, go 5/8" rather than 3/4" as the poly bushings are less compliant than the old rubber used on GTs. Plus a wagon has more weight higher than a sedan, and hence takes a different aspect in a corner.
Front end: Correct me if I'm wrong, but the GTs had relocated upper wishbones. I tooled up to do this trick on XK-XP Falcons, and am considering a template for XR-XF bodies. It's pretty basic but requires some understanding of the procedure. I can elaborate if desired. Here's some you-beaut parts to drool over (the 68-70 will fit an XY):
http://www.mustangsplus.com/Merchant...y_Code=tcpcont
Hmm... More ways to spend your hard-earned.
Adam.