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 Ĺ" 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):
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.
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.