Your car would have been originally equipped with an L28 that had fuel injection, probably an N42 casting, since that's what my 77 had in it. The problem with these (or any car this old really) is that you never really know what a previous owner may have done. Before you buy a new engine, I think you should sort out what's happening with this one. It sounds like an electrical problem, so a new engine is going to do the same thing unless you fix this one. When you say you have it in neutral, is the gear shifter in neutral, or is your foot on the clutch? Your foot should be on the clutch unless the neutral safety switch has been bypassed.