No insult intended. Do you know what you are looking for? Suggest finding a third party with a numbers matching car and see if the same anomaly exists. Better yet, have them show you why theirs is a numbers matching car. IIRC, if a fresh replacement panel is used there will be no VIN stamped into it. See if the number that exists is a part number if not a VIN or SO#.
Bottom line: If $28K CDN buys a numbers matching car and that's what you want, pass on this car until you can be certain it is as represented by the seller. If still would like to buy the car, use this as a point for negotiating a better price.
Here's a link to a decent guide. http://www.nadaguides.com/Classic-Cars/1969/Dodge/Dart-Swinger/2-Door-Hardtop-340 I find their prices are about 20% higher than prices in my area. But it's a decent benchmark. $28K CDN = $21.2K US at today's rate.