MikeT had told me they would lower it 3/4". I'm assuming it would work because the Tbird springs are rated for a lighter weight car. Therefore, the heavier wagon would sit lower. I could purchase new Tbird OEM spec springs, but I guess that's a gamble. pvan, I'll check with Eaton & Jamco. Seems like I had contacted Eaton, but don't recall what they had told me. I had contacted about 4 or 5 different custom spring manufacturers. Prices ranged from $500-$900 for a set of 4. A couple had told me they couldn't help me with the rears.
Mike may be correct on the spring ratings, but when it comes to used/old parts it's a big gamble. Your wagon also will be heavier in the rear over a T-Bird and I'm not seeing that much weight difference if you put only the front end on a scale, ride comfort maybe with a softer spring, but then if the spring rate is lower you will lose some handling/stability. You must have a spring shop close to your location you can contact, heck even on my tiny island we have spring shops......and they have taken my $ a lot over the years, with great results making me exactly what I wanted.
When I spoke with Jamco (and Eaton, but to a lesser extent) they wanted to know about use, weights, additions, ride height, etc. before they offered a solution. That was important for me as I added the third row seats, the engine was swapped from a 390 for a 429, and I will be adding a trailer hitch so I can possibly tow a pop-up trailer in the future. Heck, I had a set of used springs that I had picked up to replace the ones that were shot on my car, but when I found out I could replace bad with a known good rater than hoping the others would work out, the others went to scrap. Definitely worth the phone call, even if it ends up they can't help.
I did contact one, but they need to know specifics on my OEM springs (load, rate, etc). Anyone know where I can find this info?