Hood on my '86 300zx is wrecked and I'm wondering if a '87 300zx hood will fit as a replacement.

No, 84-86 is one hood, 87-89 is slightly different. There has been someone grafted the entire front end from an 87-89 onto an 84-86 by cutting out the metal bits in the way to fit the larger later headlights and matching hood. So it can be done but you'll need the newer headlights, and a saw and some patience. Or just buy the correct hood, at last check they were one fo the few readily available sheetmetal parts for the Z31, and ware pretty reasonably priced from most sheet metal suppliers at least check.