Months late with this thought, but just to make sure I've got this right:
The only complete or long block Z11 engines built were the 50 built for the 50 cars, and all the "over the counter" ones were either partial engines or separate heads/intakes?
If that is correct then with Jim & Jim's block dated 283 & heads 283 & 287, then this would definitely be from one of the REAL Z11 cars. Correct?
There's a good chance that somebody just found a passenger car block with the same date. The block has been decked so there's no stampings to go by.
I know the Z11 style heads were available from the parts departments. I quite possible that somebody ordered a new block and crank at the same times as the heads and just happened to hit the same casting date.
I guess at some point this was a complete running engine so it kind of makes you wonder what happened to the rest of the Z11 stuff like the intake etc.
Anyway, there's nothing that ties this stuff to one of the original 50 cars. I'm guessing it's more likely over the counter stuff.
I've owned and loved the '65 up big blocks, but STOCK performance from the best examples (375 to 460 HP) was not really any better than HP 348's (350HP) and 409's (380 to 425HP) and then the Z11 which was rated the same as L88 & ZL1 427's with 430HP. Of course those 427's (both W and MarkIV) were way underrated, but actual output was reportedly similar with 550 to 580 true HP.
I think the Z11 was a great performer. I've seen estimates of 475 to over 500 hp in stock form. That's not quite up there with the best big blocks but it was pretty good. I think the gap would have been even less if the Z11 would have had a better intake manifold.