What makes a vehicle to be considered "original"? What parts can be changed/upgraded while keeping the vehicle considered "original"?

If there is a rear end gear ratio change, would that remove the originality of the truck or would that be considered an option change? What about brakes and seat belts?

After my Dad recently died, I acquired his 1954 1-ton dually (5-window cab) with dump bed (wheat truck) that runs good and has very little rust. The only thing not original right now it the alternator system (could be reverted to the generator pretty easily). After restoration I don't plan on using it for a hauler and would like to get better speeds out of it for car show travelling, while doing so safely. Is there any way to keep this truck to be considered original while gaining a little speed and safety? If a simple rear end gear change, seat belt addition, brake system safety upgrade would take the originality title off of the truck, then to me it makes sense to go all out and modify it even more with a better transmission, suspension, etc.
Any suggestions? I am mechanically inclined but this will be my first restore project.
Thank you.