As someone who's purchased a car with a fake cowl tag, I'm not a fan. I have no idea why you want to do it in this case, but the problem can be 3-4 sellers later when you're not around to explain. That's how I (almost) got burned, but the people on the Camaro forums confirmed it was fake. I still bought the car, but for $10K less (long story).

But as to your question, it is ILLEGAL in certain US states, and it's a fairly recent law, covering anything from the OEM that lists options, colors, specs, etc. Federally I think it's OK, but the VIN is something else entirely of course.

What you can do in Ontario on 50 year old car is another story!

When I was briefly thinking about how much metal I had to replace I considered another body, but even then would have transplanted the entire firewall, not just the tag.

Other obvious problem is that the VIN doesn't match the tag, so it'd never match GM records if someone looked up the VIN. Wouldn't matter on a US Camaro, for example, but would on a Canadian car.