Bed bob question

I am going to be doing a bed bob / Supra rear conversion on my truck ( I know many of you are sick of the Supra stuff on Tacos but it's what I'm gonna do ). My question is: How much bed can I cut off without messing with the frame at all? Next question: If I was to trim a little frame off the rear, would that hurt anything ( maybe like 2 inches from where the leaf hangers are )? I was planning on adding one cross beam to the rear of the frame just for added strength. If anyone has done anything like this, please let me know. I have checked out some other bed bob threads, but did not see if the frame was messed with at all. Thanks in advance.