This movie plot hole has always bothered me too, but the novelization made this much more clear. The salesman who told Kenny that he had seen the corvette on the strip seemed to have a bit of knowledge about cars (he specifies that the paint job is "candy apple metal flake"), so it seems unlikely that he would mistake a Datsun for a custom corvette.
It turns out that the corvette WAS on display on the strip, and after Kenny told Mr. McGrath that he was headed to Vegas to look for it, McGrath called Wayne Lowry and told him that Kenny was on the way. Lowry moved a cheap Datsun in place with a similar paint job to try and make it look like an honest mistake.
In the film you can see the Datsun has a "Silverado Auto Body" sign on it, so that ties it in with Lowry.
Ah, ok. I guess they couldn't play that out in the movie because it would ruin the surprise that McGrath was in on it.