When you click on links to various merchants on this site and make a purchase, this can result in this site earning a commission. Affiliate programs and affiliations include, but are not limited to, the eBay Partner Network.
4th digit tells coupe or vert. 3=coupe 6=vert. Starting in 72 5th digit gave engine. Before 72 no engine code in vin. Corvette Black Book or Corvette by the Numbers are both good references.
You don't know for sure without the tank sticker or other documentation. There are clues though. For your car (a '68, right?) tell us the following things:
width of radiator core, and whether it has an external supply tank or if it has a fill cap on the side tank of the radiator itself
redline on tachometer
metal fuel lines from the tank to the engine compartment, one or two?
rear sway bar?
how are the u-joints fastened to the rear differential? are there u-bolts or are their caps with bolts holding the u-joints to the side yokes in the differential?