Which mods can you do to a classic car before it requires modern safety requirements?
You can replace the complete engine, transmission and driveline, body components, frame and suspension, and as long as you can continue to register and license the vehicle as one from the original model year, you don't NEED to install any "modern" safety components like air bags, seat belts or shoulder harnesses. Kind of like the old woodsman who uses his Grandfather's axe to cut down trees... Of course, he's replaced the handle four times and put three new heads on it, but it's still his Grandfather's axe.
By US federal law it needs the safety requirements associated withe the VIN as long as you are using the original chassis. States can pass any further restriction they want.
No, it will not. It will still be a manufactured vehicle from the 1960s...and therefore will only fall under the safety regulations for the year it was manufactured in. Understand, though: The vehicle may have to pass state regulations for road safety that do not fall under the general guidelines. It can vary from state to state. If the car has to pass any type of inspections before obtaining a license plate, you may want to consult with the DMV before going crazy with modifications.
Old Man Dirt
It would no longer be a "classic car"! In the long run it would be cheaper to do a "home constructed" title approach. Which might require pulling a mold off of an original. As for putting a "tesla power train" in it- that would not be cheap. I don't know if Tesla sells just the power train. If they don't then you would have to buy a Tesla to get one. Up the street there is a guy who drives a 96 Thunderbird which looks like a lead sled 1950 ford. The title says 1996 Ford Thunderbird. The reason I bring that up is that if you want a classic appearance with a tesla powertrain and have the skill to build a carbon fiber body- then start with a tesla and replace its body panels. Maybe if you design it right and your workmanship is creditable you can build them for others.
It's called swapping VIN tags and it's a felony.
Depends on state laws. VIN that says a 1966 with same basic engine, basic body means no new safety equipment since no real changes except cosmetic- chrome bits and pieces, paint. Engine swaps and major suspension mods may mean current safety standards in some limited fashion may be required- car will be considered a 'Rebuilt/homemade' and emissions and safety standards may be needed because engine year will be a determinate - California has been using new engine in old frames - split the difference in age for required equipment-- as in bumper impact absorbing mounts, side impact frame reinforcement and brake standards for inspection. Cut a frame for lowering is major change in frame- and may need upgrade in bumpers. chop and channel may need a roll bar added. SO, don't cut frame or do body alteration that changes main dimensions/height /length and likely will still be safety at time of manufacture by VIN. Watch engine swaps, check state laws regarding inspections and required equipment for 'rebuilds/homemade' vehicles