I use to work for the Ring Brothers and they built a few '67 mustangs with air ride. I believe Air Ride Technologies makes kits for these cars. They were pretty nice but didn't slam the car out too much so if your looking to go real low you'll have to do some cutting and fabricating.
I have seen a few, but none like the ones Pandy is talking about...we have the Kona project that Pandy and the crew at ring brothers built at the shop i work at...its a bada$$ even if it doesnt lay frame, you guys built some crazy cars at Ring brothers
when i went to texas heatwave '08 there was one near all the extensive stuff. It was sweet, ill see if i can find a pick of it to post, but not sure whos it was, or who built it.