in highschool shop in the 50's one of the guys cut down a straight 8 olds. He took out 3 feet. working on hot rods was forbidden. It was a farm shop class, the kid's dad had about 10,000 chickens and he said he needed s small tractor type vehicle to pull the cleanup trailer thru the chicken pens. at the end of school the "Tractor" had a roll bar, trailer hitch, seats and was drivable. A week later it had a model A coupe body with the top cut off mounted.. some how it just fit. the fastest thing in town!! when you cut down a frame you have to move the brake and fuel lines out of the way it's best to completely remove the gas tank. Sometimes the frame is taller closer to where you need to cut it. then you can sometimes slide the smaller section inside to get an overlap joint..if not use Fish plates. Reinforcing plates about a foot long on the inside so you have more area to weld. if you are luck you might find a different drive shaft that will fit or have your old one cut and rewelded. some motorhomes have stretched frames, also some of the minitruck conversions to add dual wheels. check out the junk yards and see how they do it you will be removing but the process is similar