Dave,
A 'side effect' of fitting 4 leaf springs is that your rear shock absorbers may lock out on compression. The geometry of removing leaves is that it reduces the available space, the currently available AVOs appear have a greater compressed length that some OEM shock absorbers.
I have a set of Peter Mulberry's 4-leaf (non asymmetric) springs together with Panhard rod and anti-tramp bars, and we found that the standard AVO rear shocks did indeed have the wrong extended and compressed lengths.
Fortunately this wasn't a problem, Tim Ayres was able to get a set made up with the desired characteristics, and it only took a couple of weeks.
