You can go to a few sites like http://www.pro-touring.com/ where even if they may have more Camaros etc... the techniques should apply.

They have a forum you could pose the question there and should get lots of answers.

There is also a shock/spring relocation kit (I think Jegs has them?) Mabe by Comp engineering if I remember right? But these give you about 2" of clearance...

You should be able to get an 18" wheel under there with about a 275/18/40 or 45 maybe a 50 series side wall? I know guys with a 295 under there, but usually on a 69 with a little more room.

There is always a way. And I would go with an air ride system or something other than air shocks. Or just a nice set of shocks, as you should be able to tuck the tires under with out having them stick out of the wheel well if you get the correct back spacing!

Hope this helps a bit?

Brett - 67 Drop Top 455!