Depending upon why the frame is re-stamped is the issue. If the frame VIN has been altered, then you have a frame that was likely stolen or at least, not original. It's possible that someone used a fake VIN, got an out of state title or registration, then transferred it in state (many states are very lax about vintage vehicles) and now you have it.
Since you now have a valid registration/title, it's less of an option going forward for you, but, the origin of the alteration is cause for suspicion. Ideally, your engine serial number and frame vin would point to a common year model. They won't match each other, so no issue there. If the frame is a '75 and motor a '72 then you know they're not an original pair.
Many members here are very suspicious of Previous Owners, their tales of work done, their claims, and then many become victims of false claims and hack jobs. KMB is a rather knowledgable bloke, and when he saw the stamp, his radar went off. That's the back story.
VINs should never be altered, or if they are, ideally you'd want a documented and valid reason. That stamp completely impugns the originality of the frame. But as I said, you've now got a valid title, so as long as you have it, and a bill of sale/receipt from the Seller, I'd go forward with your plans.
