I would tell you to look at the casting number behind the starter. If it is C5AE, it is a 289. (Or a C4AE, but that is unlikely.) If it is a C7, I think it can be either, but more likely a 289. (The number is the actual year of the design of the engine and the 302 was a late '67 engine, found in late model '67 as the 289 dried up and offical in model '68.)
Others should be able to offer more/better hints, but this is a start.