I recently seen a running merc for $500 with transmission.
I just bought a 1951 8BA for $100, guy advertised it for $300. I shot him a offer for $100 ... I told him to go ahead and sell it, but if not sold and before throwing it in the iron pile, I would give $100. He said come get it. .... The pics of it was in the shop, when I picked it up, he already had it back in the iron pile
It was stuck and I figured would be good parts. Still a crap shoot, but might end up being a good block to rebuild.
IMHO, a stuck flathead, is way to iffy for $400