If the sump is the same level as the bell housing you have the shortest sump off the 420 (the rover 620 sump is 20mm deeper)
You will find 250 lb spring to soft Belive me I'm on my 2nd set 350 lb, 400lb, --------> 450lb next
Zimmers = zimmerides, and is what the kit came with and they are not up to the job, mine had no resistance after about 300 miles
the bottom mounts are a u channel welded to a flat bar GBS will make you some up, if you want to make your own 40 x 40 box section with the seem welded side cut out is the right size, and this welded to a flat bar about 6mm X 11/2 that can be bolted down onto the lower wishbone, because this lifts the bottom mount you should drop to 9" springs or you may have trouble fitting them.