Need some vital help

[jrsmith_2010]

hey guys, i am in the processof bagging the rear of my truck, its an 83 longbed, and need to figure some stuff out. i need to make a notch, i am buying the square tubing from SD.com its cheaper, but i have no idea how far away from the axle i should cut the frame. then the fact of cutting the frame, whats the best way besides a tourch/plasma cutter? i have a anglegrinder with a cut off wheel on it, but will that work still? if u have pics of ur set up in the rear it would help me get a better visual. thanks in advanced!