Only dumb question is one never asked!

If you have the stock shroud, measure that and I believe it should be about 20" and therefore the 19.5" fan would be the one... and yes, the stock units had a spacer, but it may depend on the water pump shaft length. (long pump or short pump)

But if you measure from the snout of the pump/pully to the edge of the shroud, this will give you a rough idea of the length of spacer you'll need. There should only be a couple for a stock application to choose from, so you should be able to determine it that way.

The fan should go into the shroud, but just to the point where the back of the blades are about at the edge of the shroud...

I'm sure some of the others will have some more difinitive info for you.

Brett - 67 Drop Top 455!