What I would do is take the springs completely out and then block the chassis at the finished ride height..Then I can see just what I need to do to get there from here..
the minum ride height that I will use is the height of the tire sidewall..If you are planning on using some of those 35 or 40 series tires then the car winds up with about 2-3 inches of ground clearance..Plenty low enough for even the most hard core low rider..with a 70-75 series tire you will have about 7" of ground clearance..
Now if you are using coil overs the distance between mounting points is 12" on the front and 14" on the rears..
You can go to stock car products.com and get the springs that you need for reasonable last time I checked..and check out their tech on chassis setup as it is very educational....
Also if you can find any of the books by Carroll Smith on suspension and chassis they are well worth the money..try Steve Smith autosports.com for those..
I have tried most all of it and now do what is known to work..