Slant six spark plugs and numbers

The plugs I pulled out are autolite 26. There is nothing in the NAPA stores, or any other parts store around here, computer that lists these as the correct plug. All the recommended plugs have a longer thread, and a 13/16 hex, with a gasket. There is no room in the recess that the plugs sits in for that size socket. The 26 plugs are 5/8, with a short thread length, and no gasket, but I am not sure if they are the correct plug, or just what the last person to work on it could find that fit in. I found another brand of plug on line with a longer thread, but I don't want to use them if they are going to be too long, and hit the piston when it comes up. They are Denso 4501. The guy at NAPA seems to know a lot about old Mopars, but he doesn't know what is going on there either. My confusion comes from not being able to find any recommended size in any store that will actually fit in the engine. So I don't know what I have there. I assume its the stock engine, but being this old anything is possible. Does anybody else have trouble finding spark plugs for a slant six? Some people on here talk about NGK 3459 plugs, but I can't find any such thing available.