Axle to frame clearance

[t-tung]

I'm trying to get the rear axle situated right now and need to know what's the minimum distance between the axle and the frame/bumpstop I can get away with without it bottoming out every time I hit a pothole or bump. I'm running the Explorer springs in the rear (spring under axle). Also, do the spring eyes need to be level or is okay if the rear eye is well above the front eye? I'm no engineer, obviously.

[t-tung]

Springs will be outboard of the frame and plan on running the shackles below the spring in the rear as on most chevy trucks.