Realistically, you first need to know the rear weight of your car... each side (L-R), with driver and how you prefer to be driving the car (w or wo spare tire, trunk stuff, full tank-3/4 or 1/4 or less, etc).
Do you intend to occasionally weigh it down with a full trunk load... ice cooler, tent, travelling, bags/suitecases, tool boxes, spare parts box, extra race tires, etc?
The more arch remaining after weighted down.. the worse handling you'll have. Arches in the rear leafs welcome greater lift when cornering. Flatter leafs (w/minimal arch, such as ~1" - 1.5" compressed arch/maximum.. better yet near flat/zero arch... help keep the rear of the car impressed into the pavement.
Weigh the car (as you'd be most often driving it)... all four corners... then try to balance the corners (left-right)... TBs up front are easy; rear requires shims under the axle... then re-adjust as needed... front and rear. Also be certain the sway bars are disconnected at the links so to avoid pre-loading those corners.... shim them back together after corner weights are finalized.... with the driver seated and the car weighted as you prefer to most be concerned of driving it (street/hwy/cruising... or competition).
A 250#/5" arch will require 1250# of compression to become near flat... the rear of our E-bodies is mostly about 750-900 max at each wheel, pending your weighted vehicle. Don't make a decision until until you weigh the car.
