Proper way is to use De-cambered springs .. or get yours done.
If you use the undermount idea, you may need to cobble up the plate-to-axle and weld to casing correctly. Not as easy as it sounds. This locates the axle to the spring correctly.
Things to be careful of are .. Bump stop clearance (suspension travel)
Damper - Range-of-movement.

For the record, I have a Mk4 with decambered springs. Dropped it 4" ... and the twin JL 15" W3's and a few amps dropped it the extra 1"

I had to modify (rework) the bump-stop plate on the chassis rail to give an extra 1" of travel before it contacted. Otherwise, it will go solid suspension most of the time!