I think the evaluation of the motor has been fair and up-front. Good chance it is in fact a NASCAR engine. How could you prove that to a prospective buyer? The motor IS worth a ton of money, NASCAR or not, it would be prudent to learn as much about it as possible.
Is it worth 25K? Well,,,,,,,there could be room for some "flames" over that amount. Not from me, I don't know about "values". BUT that is a BIG asking price and it's bound to draw some fire. Better to discuss it here and now than stutter and stammer when a "real" buyer shows up.
No intended flames from me, just facts as I understand them (and I COULD be wrong)!
It's a serious proposal, the sale of this motor, it deserves some serious attention (good or bad).
Ernie
EDIT:
No way I would "part" this motor out! If it is indeed a NASCAR motor and there is some to verify that it is worth far more as a complete unit than it will ever bring in pieces. The KEY is how to PROVE what you got (not to me, heck I believe you, "cool").