If you go to the 3/5, the first place that is going to cause problems, is your box crossmember, right above the rear-end's pumpkin. Some people remove the crossmember. Not a good idea, if your going to use the truck, as a truck. I notched mine. I'll post some pix when I get 'em off my camera. (stupid busy with work, right now) You will have to relocate the brake line, where it's mounted to the top of the pumpkin, so it doesn't get pinched there.
The next place to hit, will be the frame rails, right over top of the rear-end axle tubes. You may need to do a mini-notch on the rails. Dont forget to make provisions for the mounting of some urethane bumpers in there. I see that 'Air-Ride' has some progressive compression foam bumpers, now. I haven't tried those, but have heard good about them.
I strongly recommend a rear sway bar. Totally changes the driveability of the truck.
When I first had my 4/6 drop installed, the truck was damn near impossible to drive, due to the geometry changes with the shock positions. Air-bags completely cured the problem of bottoming out. I actually bent the floor of the box, but it went right back, with a 'pop', when we cut it for the notch. Sure, I could carry 5000lbs, but I didn't like what I was getting, performance wise. Ended up ripping everything out, relocated the upper shock mounts, lost the bolt-in C-notch, & boxed the frame with a proper custom notch. Custom sway bar mounts & end-links. Now the suspension works the way that I want it to. Probably more than what you require, though.
