Your 68 will not have 17 numbers. My 70 Stang only has 11 or so. I don't have the link (cuz I'm at work) but type in mustang decoder on yahoo and there are several out there. You will be able to learn all the original specs (ie: engine size, etc.) Here are a couple of sites: http://www.mustangdecoder.com/decoder.html
That's because VIN numbers weren't introduced until '81. Before that there's no way to track a car like VIN number do. So yeah, you'll have to take the seller at their word. You can also take it by a mechanic and have them look at it to see if it has any problems. Sorry.