Mick,
We did the add a leaf to the Outlaw DennisH's car, and it worked great and has been consistent ride-heigth wise.

It was from summit and originally intended for a 4wd truck. As I recall, it was about 18" long.

Either that, or I have several sets of passenger side SS springs you can try, which is what I run on both my cars. They both weigh 3700 lbs, but I found that the 3200 lbs passenger side springs work best for my needs. I did have to add a modified set of slapper bars to both cars, as the power was wrapping the front segments. They now ride and hook better than anything else I've tried.