If the rear's already plenty low and looks stock, my guess is the leaf springs were reworked at a spring shop or something to get it lower. How are the shocks mounted? Are they mounted in such a way that they'll be effective? It's my understanding that (at least with coil spring Chevy's), if lowering the rear, it's necessary to relocate the shock mount to regain travel. I don't have firsthand experience but, am getting ready to find out when I install my bags. You may be right in that a good set of shocks (effectively mounted) would get you where you need to be for the rear. How 'bout a panhard bar, adjustable trac bar?
If you go with drop spindles, you're not really swapping for later model parts, it's just what the spindles are made to accept. As far as the conversion goes, don't know much about it as I haven't been and am currently not interested in disc. Besides the parts already mentioned, I think a dual master cylinder is in order. Sorry I'm not much help on the conversion. I'm sure if you researched disc conversions on this site, you'd probably find all the info needed. There's probably also plenty of info on the net. Just do some creative Googling.
-David
