Something to consider, if it has two inch drop spindles and you drop it another two inches the front crossmember is only going to be about 3.5" off the ground. That's with a 12" rolling radius tire. Might make things interesting in the header and oil pan clearance department.
Those numbers are taken from the information on our g-Street bolt-on front suspensions:
http://www.cachassisworks.com/c-556-...gm-f-body.aspx