Well, I've got the pictures. There's 36, but I'm going to just post ones I think have the clues to what the car was.
First gives an overall idea of the car as it sits today. It has new EDP coated front sheetmetal, including cowl hood. I can't tell if the sheetmetal is repro or GM. It isn't fitted well at the moment, but I can't tell who made it (would have to look for GM stickers).
Car has a 12-bolt, been in there a while but can't say it's original. Has U-bolts, slapper bars, and 5-leaf springs. Two fuel lines, so it was a Quadrajet-equipped car, which means it could have been an LM1 (350/255), L48 (350/300), L35 (396/325). or L34 (396/350), but not L78.
Not a lot to say underhood. Heater box cover is out, it does have the speedo hole near the distributor (can't see if it also has one near the fuse box, to see if the upper hole is original or not. No wiper motor installed, so no RS headlight washer solenoid visible. The firewall has been drilled where the 6-cylinder throttle cable would go through, but it has a V8 VIN so I doubt it was a 6 cylinder car originally, unless the hidden VIN under the circular hole for the heater fan doesn't match the tag on the dash...
This is the only shot I have that shows the radiator, and I think it is a small block radiator and shroud but can't be certain.
Dash carrier looks like a new repro, because the tach (which also looks repro) doesn't sit in it yet (they don't repro the inspturment panel for tach applications yet, so you have to cut the back of the carrier out to get the tach to sit down in). Tach is a 5K redline; if it was an original tach belonging in an original L78 car, I think it would be a 6K redline. 5K redline had smallblock applications I think, but again it doesn't look original. I have a shot that kind of shows the underside of the dash near the fues block, and I don't see any vacuum hoses for the headlight (RS guys, I'm not certain where the lines go through the firewall to the switch; can you see the area in the above driver side underhood shot??)
Just for yucks, here's a shot of the $10K 396.
I don't have any shots of the RS reverse lamps where they come through the tailpan, so can't say if they look original.
Vin, please don't take any of this as a poke at you, but I don't see anything here I would remotely pay $20K for. Maybe someone with data would have an idea which Q-Jet equipped cars were rolling down the line at Van Nuys the first week of February 1969, but this is definitely not an L78 car. There could be a build sheet still above the fuel tank, but I kinda doubt it. If it does not have the reinforcement plate for dual exhaust on the driver side rear frame rail, then it was definitely LM1 without optional duals.
The 12-bolt could have been in any of the engines listed above, as could the 4-speed (an M-20 most likely, possibly an M-21 if big block and low rear gears).
Might have been an RS, but without seeing the tailpan (and without input from the RS owners about whether you could see vacuum line holes in any of the above shots if it was a factory RS).
As far as the engine goes, it looks nice if you like that look, but it is obviously not remotely correct looking and won't really add to the value of the car the way you think (IMO). If it is a rare block and components (what's left that's GM), maybe it's worth something to the Chevelle guys like the Z motors are to the Z guys, but they'd have to "restore" the appearance of the engine at a minimum.
From everything I have seen, I think that car as it sits now is about $10K overpriced, even if it was a real RS and possibly even an SS. Without the matching drivetrain, it will be hard to prove it was ever an SS. A solid, completed car with these parts is a $25K max car, I think.
Anybody else want to wade in here??