Hello,
Depending on your current dimensions, you are going to either have to get a real small sleeper, so you do not go over the max length restriction, or get a larger sleeper, and get a different box.
Personally, I just recently took a 28 foot box off my day cab, dropped in a 72" Alumi Bunk I bought for dirt cheap, and in a couple days, I am driving it down to Thurman, OH to have the box dropped in place and mounted.
The person doing the work is Darrell Marcum- his handle is "Toes" here on EO. This guy has given me a tremendous amount of free advice, and because of this, I was able to remove my old box myself, and mount my sleeper myself- with the help of a friend of course.
He has access to the hard to find 22foot by 102"X102" used boxes.
If you want his number, send me a message, and I will give it to you.