I share your concern about the SLC not being a replica. I think you should download the
ASSEMBLED AND REBUILT VEHICLE MANUAL and read Chapters 2 and 4 very carefully. On the face of it, they make allowance for assembled vehicles that do not represent an actual make. But the fish hook is: unless you register (separate step from title) as a "Custom Vehicle" you will have to get a 2015 compliant safety and emissions inspection annually. That's why I suggested that you just pick something.
The definition from the manual of a "Custom Vehicle":
A custom vehicle is a vehicle that:
• is at least 25 years old and of a model year after 1948; or
• manufactured to resemble a vehicle that is at least 25 years old and of a model year after 1948, that has been altered from the manufacturer's original design, or has a body constructed from materials not original to the vehicle.
I suspect you may be the first SLC or GTM to come along since all these regs were published in 2014.
Sam