Another lwb to swb

[Mullinsracing]

I'm planning to begin the shortening process of my newly aquired 71 c10 tomorrow. I have been reading up on this and have a question. I am going to attempt to cut down the bed instead of buying new bedsides but am seeing 2 different dimensions being used. I know that I need to remove 12" out from in front of the rear fender wells since I will be removing 12" from the frame under the cab but how much off from behind the rear fender well. I have seen some say 6" and others say 8". What is the correct amount to make the bedsides match that of the swb?

Thanks for the help.