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The engine stampings are good, and indicates a later conversion
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Marlin what makes you say this? If the stampings were on a vintage 427 or 454 yea maybe, but on the original 396 block? The heavily documented '71 BM Camaro that I owned that retained its Motion built LS-6 did not have these stampings on the block or heads. Does Joe G's 396 BM Chevelle have these engine stampings?
Secondly, it's just illogical that the car is advertised on Ebay as a likely Motion modified car and the Seller won't pay J.R. $2K to potentially reap an extra $100k+ in value after documentation. Doesn't make sense. Perhaps someone will take a chance and it will pay off.
I recall several people on this board were looking at the 68 Yenko when it was for sale a few years ago and there were apparently some issues. If it now turns out to be real, that's great. I'd like to hear more about it.