i have done this 5 or 6 times in the past , but i cannot remember the amount to take out .

remove the bed , behind the 1 bedbolt cut the frame off and remove , i think 9 or 11 inches from the frame . then remove 4 inches off the rear of the frame . and reweld it slowly because it will warp a frame real quick , add gussetts also inside the cuts .
thats the quick version .