I have a '76 and a friend of mine has a '77. They both use the same spark plug. I don't know for sure that this is the plug you're supposed to be using, but I'd be willing to bet it is.
NGK - BP5ES
Now, on the top of the cylinder head it is stamped that the plug should only be a 3/4" plug. So be sure whatever you put in it, it doesn't stick into the cylinder too far. It will impact the piston and do substantial damage.