Let me start by saying that there can be a LOT of confusion regarding the subject of head gaskets for 5000s.
You say new pistons and liners. I hope you know that liners were not factory installed. Is this now still a 4.2" bore engine (233 CID), or a 4.4" bore engine (256 CID)? If you are using a 4.2" bore head gasket with 4.4" holes, I could see where a problem would exist.
Also, with regards to 4.4" engines, there are two different head gaskets. One is thick, and one is thin. Thick head gaskets are used when there is more than .010" of piston protrusion at TDC. Thin gaskets are used with less than .010" protrusion.
Now, go to the NH parts page and tell me you aren't even more confused. It only differentiates by year. It doesn't even show a gasket for a 1965 model.