Hafta say I agree (strongly) about not relying on the orig. wheelbase dimension for a lowered car, if that's what you're looking for. Of course you can't load the suspension up so early in the process but instead leave the springs out and use 5/8 threaded rod or whatever will fit in the shock holes, plus whatever you would have to make such as a welded-on "T" tube or just a big nut to fake the lower shock mount, and tie everything together tight at what the eventual at-rest geometry should be (i.e., lower control arms level as measured from pivot points). Mock it all up on the floor to the ride height you want, using the tire-wheel you plan on, and the crossmember at the correct angle etc. and stare at it for a few days to make sure it's what you like. You will be doing a lot of fab work and kinda returning to how it was done before all the ready-made crossmember kits came out, which can use the MII parts and take much of the figuring out of the deal if you want to just pay the cost. Keep in mind an original MII suspension can be a pain to chisel/drill out of it's frame and was designed to work with a 13" wheel and may not be adjustable back to the castor angle you'd want, you'd have to check on that yourself, with a kit you can get whatever. BTW 5.5" ground height at the rr bottom of the front fender to the ground is a nice practical number if you do plan on having it lowered.
Do a good job of this and you'll love the results.