Autolite 26 is a 14mm, .460" reach, 5/8" hex, tapered seat, power tip suppressor spark plug.
That is the second hottest spark plug available. There are only 2 hotter versions. Neither in the power tip. And the AP26 you have, the "P" is for Platnium. The fancy exotic metal tipped spark plugs are great for moder EFI engines, but old carbed engines they are a bit pointless.
OK, the hotter plugs. autolite 147 or 687 are the only hotter options that will fit your heads. The 687 is the wide gap version. Stock application is a '75 Checker.
All thanks to the phone book autolite catalog. The real paper catalog, not the online stuff.