need help with spark plug choice and tapered or flat

[dmjlambert]

Here is a picture of a spark plug my engine builder put in, and one of the hole it fits into. I have a 1969 CST/10 with 350 CI engine, the original engine rebuilt for daily driver needs (nothing special done to it). I was told the heads were not re-used so are not original to this truck. Is this a taper plug in a flat seat hole? It looks that way to me, but I just don't know. The spark plug has Autolite 26 marked on it and I do not see any other markings on it.

I was getting ready to remove the plugs and check if they are OK and gapped ok for HEI, and I found the first 2 very loose.