I never have had anyone want to check the frame or chassis here on any of my trucks,even on my '72 Chevelle wagon with a '69 Suburban K10 frame under it..
My theory is no one had to know the car no longer sat on its original frame,and nobody asked...I paid for my registration and plate swap fee,and I was on the road...told them its a '72 Chevelle Wagon,period..they didn't need to know it was now a 4x4..or modified...
I had an '85 cab & doors on my '77 GMC K2500 ,the newer cab had a good title so it was registered with that title and numbers before I had bought it--the former owner saved the original tags and title off the old cab,and also gave them to me "just in case",but no one bothered at inspection time to give the truck a second glance..I owned it over 15 years and never had anyone ask or look close at it..
Its unlikely there would ever be a problem unless it was suspected the parts you swap on were stolen,then they might dig deeper--but body shops fix wrecks daily,junkyards sell doors & fenders with just sales slips that have no vin on them,etc--there is so much of that going on its practically impossible to get hassled about that..on something as old as a square body I'd guess most places dont care much if the truck has all its original parts..if it were less than 10 years old then yeah,red flags might get raised..