I have an ongoing dakota drag project.
we didnt shorten the frame, but we did box the inside of the frame from the factory boxed section to the rear bumper with 1/8", it added some weight but did it ever stiffen up the frame before it was a flexi flyer now it's like a battleship !!
I wouldnt hesitate to cut the frame straight, bevel,butt weld and box the frame on the inside just make sure you secure the frame. our's was fixtured up on a surface plate and allowed to cool it was stitch welded over a two day period to control heat induced warpage.