Clarification on a few points

If you took the "hybrid" out while you were changing parts over then the "hybrid" would certainly be a derivative of the Model T and the Kit-only car a derivative work of the "hybrid".

If you built your kit car while looking at the Model T and never depend on it for either structural integrity or functionality then it would be more complicated. And obviously if you went further still and only took the spec of the Model T and reimplemented it from that then it would be "inspired" and count as clean room reverse engineering.

The real problem is that this isn't a car and the term "derivative work" has real legal meaning and precedent in the world of copyright.

I'm not saying Bruce would have any claim on Toybox purely because Langley has seen Busybox, but I would be careful.