Here's my take on the car....for what it's worth. It is a beautiful car, so don't take any of this the wrong way. A missing fender tag is "fishy" on such a rare car.
1. If you are paying big money, then show the owner your cash & demand to see the VIN numbers on the radiator core support & the trunk lip, under trunk seal, up close to the top driver's side corner behind the window...at least that's what "I" would do.
2. The birds on the fenders look a little far back, so I strongly suspect it has been repainted, nicely, but repainted.
3. Take a very close look at the dash (it looks restored) to see if there is any welding on/around the dash frame. I "think" it was just Roadruners & GTX's that got the rally dash, so if it's original you can rule out a Satellite or Belvedere (somebody correct me if I'm wrong on this). It can be changed, but it's a pretty big job.
4. A 440+6 spd car should have a Dana rear end
5. Any 440+6 car (or Hemi) should have a skid plate under the front engine cross member.
6. Does it have power steering? I "think" a 6pack car would have a power steering cooler built into one of the lines.
7. The radiator would be an 056???? (other guys here know WAY better than I do) if it's an original 440 car.
8. The last number of the VIN should be stamped on the block, I think towards the front of the engine, passenger side, right above the oil pan, on a machined 1/2" x 3" pad.
9. It should have an "offset horseshoe" in the center of the harmonic balancer (70 six packs are "big rod" motors)
10. I think the last numbers of the VIN should be on the transmission somewhere too. It also should be an 18-spline input shaft vs. a 23-spline, but I personally don't know how to tell them apart with the transmission in the car.
SIX PACK EXPERTS PLEASE CHIME IN & CORRECT ME WHERE I'M WRONG.