This is the frame that is going under the truck I am stretching the cab on.
The extra front crossmember (this is a GMC 1/2 ton frame) is where the added length is.
I'd shorten one in that area just in front of the crossmember with the gussets on it.
And I would overlap it per one of the board members diagrams from another thread.
Note that the measurements are off for the AD trucks though and I think the difference in length is 9-1/4 inches but it may be 9-1/2. The overlap gives a lot more strength than a but welded frame would. I'd still box it or plate it in that area though.
I'm pretty sure you are going to have to relocate the center running board bracket too but don't have the boards off the 48 to measure the difference at the moment. If you were closer I'd swap you a pair of nice short bed running boards for a decent set of longbed running boards.