Pretty sure 80s boxes are just sleeved through the frame, so you’d just be cutting a chunk. Box it, drill, & sleeve … will have to figure out the shock tower though.
Tucker
Tucker is correct. No bracket needed. Drill holes, insert DOM, weld the DOM in the hole. Done. I'd recommend putting a plate on the inside of the frame when you do this to add more strength if you wheel it a lot.
Do the steering before the body work. You'll need to trim the inner fender to clear steering lines once you put in an 80 box.