hello 58chev.
i am getting ready to order this flip kit for my 57 from summit.
http://www.summitracing.com/parts/DJM-FK1024-5/although i have a 1980 rear in my 57, it will work the same for your 58 im sure.
you keep the factory mounts that are welded to the rear housing, put the supplied U bracket into the factory mount from the bottom, which will let your leafs mount under the rear. depending on your axle diameter, you will get around a 5-6 inch drop and keep the original ride. mine will drop 6 inches when i install the kit. my axle housing diameter is 3 inches.
but, you are probably gonna have to "C" notch the frame, as waldo53 said.
i already have this kit,
http://www.classicparts.com/1955-59-Rear-End-C-Notch-Kit/productinfo/93-315/but am waiting until i get mt flip kit before i take the truck down.
i installed a flip kit on an old 1974 chevy i had years ago, and didnt do a c notch, and on bad bumps it would bottom out on the frame. not good.
hope this helped.
leonard