Thanks for your reply, I think you're onto it.
Here's a pic I pinched off the net. You can clearly see that the plugs are angled toward each other. Earlier Chevs had the plugs at right angles to the heads but aftermarket heads are available in either straight or angled (also called slant) configurations. It's something to do with the spark assisting in the combustion process.
Anyway, if your '87 is stock and has angle plug heads, then you've answered my question. Thanks again.
BobW