Welcome. I hope your search pays off tomorrow. Hawaii is right this is the greatest group of guys around. Everybody is very helpful and there are no questions you should be afraid to ask.
There is no way by the trim tag on a 68 to verify what you have other than if the car came with a V8. A few quick things to look for. 12 bolt rear end, multi-leaf rear springs, the cars had dual exhaust so there must be a welded on ddoubler on both rear frame rails just behind the rear tires. Look for a 3/8 fuel line. If it was a big block SS it would have originally had a black painted tail panel between the tail lights. Pull back the weathertriping in the trunk to check for any old black paint at the very back edge of the trunk opening. Take the gas cap off, it should be chrome and black and say SS on it. Look to see that it has the cable that retains the cap to the tail panel of the car and look closely at how it is attached to the filler neck where the calbe attachs. You should be able to tell if someone has added this gas cap or if it looks correct. The grille should be black and not painted black, but molded black. Look closely at the back side and scrape at it a bit to see if the black finish scratches off or not. Look at the heater bos under the hood. If it was a big block car the hoses will enter in the middle of the box, this is a sure sign of being at least an SS big block car. the small block SS cars look like all other heater boxes. Beyond these things just about everything else that is quick to check was optional.