nah, a body lift shouldn't be needed. i am really surprised you are having problems with 35's and the 6" lift. I have plenty of room in the rear with my 5.5" blocks, enough to where i could probably run around on 38's on the street just fine. The front is very different though, under a hard flex i can get my 35x12.5 bfg's to rub the fender by the doors, and they've done a little body work for me already (natural fender trimming, they take what they need ;-) Seeing that it's an 85, would you be willing to trim the fenders and put on some flares? I've been thinking bout doing that to my 93, taking out as much sheet metal as possible and putting on some good looking flares. 6" lift is about all i want on my daily driver, but if this truck ever becomes my full-time mud toy, hang on it's going HIGH. One other note, you can do a shackle reversal in the back using mid-80's f250 hangers and it's supposed to give 5 to 6" lift alone, so you could then add some blocks if needed, but I doubt you would need to. It would cost less than $50 bucks and wouldn't change the ride quality.
I would also like to see some pics of your rig on those swampers, I've always liked those 35x15.5 looks and I bet they rip the mud up like no other. I'm considering doing a solid front axle swap, cutting the fenders, doing 5.13 gears (i've run the math, it ain't bad with 29% OD), and putting on some 38.5x16 swampers (maybe radials) with some lesser backspaced wheels (these wheels rub like a mofo already). I need to change gears anyway, so I'm thinking if I put in 5.13's with the 35's and decide it's too low, I'll just have to "fix my problem" by putting on some taller tires. Makes sense, no? :-)
S H O U L D A G O T T A F O R D!