69 LWB drop

[Pete357]

Hi guys, new to the site here. Ive had a 69 gmc lwb for 3 years now that i am finally getting around to working on this winter. Looking to lower it enough to get most of the air space out from between the fenders and tires with out worrying about tire rubbing fenders. I am running 215/70 R15 tires up front and 285/70 R15 tires in the rear. Just wondering if anyone has an idea of what kind of drop i can get away with? I know i have atleast 5" to spare in the fenders, does this mean i would be safe to go down 4"? Also i see its pretty standard to lower the rear 2" more than the front. Is this just to level the truck? I kind of like the rear end up like it sits now, would this mean i want to drop it the same in the rear as the front? I was planning on using the grand slam kit from CPP including the disc brake conversion and upgrading to the modular spindles, any one else have any different suggestions?

Any help is greatly appreciated, ill try to figure out how to get pictures of it up here soon.