Tell him "look, it's a Malibu" (always true, right Dale?) just to get the discussion going...
If he whips out the Canada docs proving it started life as an SS454, great - but he won't...because chances are 50/50 it did not start life as an SS454, and he's already said "no paperwork" which is a nice polite way to say "SS clone"
If he still insists it is an original SS, again ask for specific proof. He wont have it. Ignore the legends and anecdotes about "the guy who built this car is an expert..." Blah blah blah...
After his schpiel is over, courteously point out "Look, we can agree this is a nice car in resale red that has some initial curb appeal...but we must also agree based on no documentation and obvious features that this is a resto-rod that very likely started life as a small block Malibu non-SS car."
If he STILL won't come around on the "probably not an SS/resto-rod" thing, begin to cite stuff like stripes which are the incorrect color, car has headers, non-stock exhaust, aftermarket intake manifold, etc. Nothing wrong with those features if you want them, but that makes
it a resto-rod...not a concours restoration.
If he stonewalls and says "what difference does all that really make?" Courteously point out that he's asking for a price which is tilted towards the end of the scale "more representative of an OEM restoration car which can reasonably be identified as an original SS"...and that such a position is just not realistic for this nice resto-rod probably SS clone in resale red (which is likely not even the original color on the car, by the way).
So, having done the above, now
the bargaining begins.
When he waffles, offer him mid to high twenties... Of course you won't GET it for that, but if he won't move, IMO - walk away.
Use your brain, not your lust for a Chevelle. This is not an "investment" car...it's a fun car, a nice car, a sharp car, but at $40k before sales tax, it is not an investment car. If you buy it at $40k plus sales tax, then YOU are providing the FINAL PAYOFF. $40k is the high water mark in what is a long string of "other investors".. YOU are the last guy in the Ponzi scheme...IF you are thinking "investment".
But, if you have the money and don't care about the above, and just want a red big block Chevelle to drive and in which you can have fun? Perfectly fine...that's your call. Just tell yourself the truth as you buy this car. You will rest easier knowing why you bought it and what you bought. More significantly, you won't have to lie to yourself, your significant other, and to the guy to whom you sell it five years from now. As the man said...
"To thine own self be true"