Thanks for all the feedback.
I'm not really sure why I'd need to reset the springs since it is already low enough, or is that to compensate for the extra height gained by adding a leaf. On that note, why does adding a leaf raise the car?
My front crossmember is around 80 - 85 mm ground clearance so I'm going to need a cert at this height regardless. I'd say no matter what I do I'm probably going to need to either massage the floor, or more likely, cut the floor and have a new section welded in. If I can do this and still run my original rear seat then that shouldn't be an issue.
Cheers