Exhaust Manifold AIR Plugs - 85 Burb 454

[gofast321]

I have an 85 Suburban with a 454 that I recently purchased. The AIR ports in the exhaust manifolds have been pluged with fittings that have a pipe thread to them and a flat top that allows me to tighten them down like a large screw. Recently one of those plugs has backed out and was lost. I have replaced it with an NPT fitting. The problem with the fitting is that it will not allow me to tighten it down because it has a continuous thread and consequently it keeps backing out and I have to constantly replace it.

Does anyone have any suggestions on what I can do to either keep the NPT seated or where I could find a plug similar to my other ones with the flat "screw type" top?

Thanks for the help!