A SPF or Kirkham is a finished car and comes with an MSO just like a Ford or Chevy. If you buy it from a licensed Texas auto dealer the law says they are required to collect sales tax. If they didn't collect it, you are still liable.
If you buy a body, engine, tranny etc and assemble a car, you are not supposed to pay sales tax when you title and register. But that is because the state is assuming you filed forms 01-156 and paid the Occasional Use Taxes discussed here:
Occasional Use Tax
Did you file those forms on your components?