Slant six spark plugs and numbers

Hi. I'm new here so bear with me if I get something wrong. I have a '73 swinger I just started working on. I was looking for spark plugs, and all the plugs for this cars 225 slant 6 at the store have a 13/16 hex size. They won't fit, as there is not enough room for the wrench in the cavity where the plug sits. The ones on it are 5/8, but I don't know if they are correct. They are short compared to what the stores say should fit. I can't find numbers on the engine to see if it has been swapped. Is it a thing with the slant six to need a 5/8 hex plug? I have always had V8s before. Is it possible the engine is from something else, and what would require a 5/8 hex plug? No numbers in the places folks say to look that I can see. Any help appreciated.