Revell Slant Six model, unbuilt, sealed bags, all there. WIW?

And, includes one of a kind paperwork from Bill Weertman, creator of the engine.
The documents are about the corrections required before the model could be made for production.
I plan to sell it and am trying to put a value on it. I worked for Chrysler Engineering 37 years and bought this from a guy in the 80s that worked with Bill in 1961.
(that's how he got the documents)

Any input appreciated.

Thanks.