Only thing thats making me wonder is that i see no Yenko emblem on the tail panel... the pic is blurry, but it looks like the stock Chevrolet emblem in the center. Yenko *usually* replaced this with a Yenko emblem, (although i'm sure there were exceptions...) and each fender should also have one... They were the stick-on type, so there wouldn't be any holes in the fenders to see where they were removed if they were ever there...Check for evidence of a vinyl side stripe, if the car has been repainted there could be evidence of it still in the jams and fender lips. If it has any of this stuff still on it, i'd say its likely legit. The valve cover is definitely a real Yenko piece, and was actually cast by Atlas Foundry, the same place that made the 5 spoke mags for Yenko in 69. But...as stated earlier, these covers were available through their catalog too though, so it may just be an add on to a standard Vega GT that someone ordered years ago. I do believe all Yenko Vega's started out as GTs. From what i see so far, I'll give it a "definite maybe".
I notice the VIN is earlier than any posted in that list, but I also notice the VINs on that list seem to be all over the place as far as sequence. Could be legit...