So what did you end up doing? I was looking into adding a Air over leaf helper to my wagon, would love to find some pics of any. I wanted to tow a little Shasta with the wagon (59 Edsel) and still keep the third row, and like everyone else keep the cost down, but still use new reliable parts. This is what I was thinking http://www.ridetech.com/store/airoverleaf/
Leave the old springs to locate the axle and add air bags for adjustable spring rate. I've been doing that to all the pickup trucks i've owned. Works great. Keep a small bicycle air pump in the car and add spring rate/height when needed. When the loads gone let the air out. DO NOT connect the bags together on the same line. Keep each side separate and you get some anti sway as well.
Thought I'd chime in on this thread. I would also like to hear more experience on the matter because I am after a similar situation. I don't mean to hijack - there's just no point me putting up a new post with the same question. My 53 chevy is my daily driver. I would like to get the back end sitting a lot lower, but keep it on springs for rock solid reliability. My driveway sucks though, and every time I go up it I scrape already (and the car isn't that low). Would it be logical to have a system where you run leaf springs and just have air assist bags in the rear to raise the car to get over steep driveways?