I'll give you a hint why your potential buyers seem ungrateful. As the car is a non-numbers matching 350 2bbl, auto trans, buyers see it as such. They don't care that you've spent $25,000 on the restoration. They know that 350s aren't "worth" as much as 400s, that autos aren't "worth" as much as 4 speeds, and that 2 bbls aren't "worth" as much as 4 bbls. Same issues I had with my Le Mans. Same place 4 door Tri 5 Chevys were 15 years ago. Now 4 doors of some makes fetch good money where at one time you could scarcely give them away.

The money spent on finding and rebuilding a correct YE code will not likely ever be regained. Dropping a 70s era 400 would probably bring more return on investment. It does not make sense from a restoration standpoint, but it shows in the market. People want what they want, and the 400 is supposed to be "the" engine. 350s are considered the low end, even though a good running 350 is no slouch.

Your buyer will likely be a family or older couple looking for a nice family cruiser. And it will fill the bill admirably.