How do people get Rebodied classics onto the road on a modern chassis?

depends on exactly how your legislation is written but from what ive seen you can do quite a bit to a vehicle as long as it will still pass a certain kind of inspection. i cant say with any real certainty for this particular modification but ive read in cases where extensive stuff like complete driveline replacements were done you would get a "specially constructed" title and an enhanced inspection. in most cases if youre not causing any major issues and are playing things cool, if your vehicle is safe, you will probably be fine. if i was building a vehicle like this i would probably ask a lot of questions and make sure i was getting the right answers but i actually know how to do stuff like this safely which should definitely be your next biggest concern after legality. having done motor swaps and huge suspension conversions etc. that dont really change what the car actually is, i personally just take my vehicle somewhere the inspector will be chill and i end up legal. if you find a real asshole tech they might fail you and recommend you some changes, if you didnt build your vehicle safely and drove it in they might actually flag you and make sure you cant drive it anywhere until you fix everything, they also might just do that if theyre a real asshole about the rules. there are a lot of variables here and i feel like most of these questions would be for a notary, not an engineer. the notary will either have the answers you need or the resources you need to find them and generally people at notaries dont really give a shit if youre trying to break the law like the DMV will.