No the rj series is still a 13/16 hex. It just has shorter threads so the electrode end won't reach down far enough into the combustion chamber. The rj plug is what the big block engine calls for and "if I remember right" the older poly series engine as well.
I think that you want "re" prefix plugs if using champion. There's 2 that will work. One listed for the magnum series engine til 96, the other listed on 96- newer magnum heads. The difference between these two is that the 97+ version has a longer ground electrode, the inside insulator seems longer to compensate, (it isn't but sets into the threaded portion farther)
I remember when my kid had his 90 w250 with a. Gas 360. He had gone to the parts store and asked for plugs for it, and they gave him the plugs for a magnum, I went back with him to return them for the regular *RN" series plugs as LA motors always used and they showed us that "their" listing showed many of the LA engines had been superseded to the newer style peanut plugs.
So with that I called the Federal Mogul (Champion) tech line myself and was told that besides the difference in wrench size that these peanut plugs were identical to the original plugs that we were used to seeing in the LAs and could use either/or. The main reason why magnum engines went to the different style plug was because of the pressed in tubes around the plugs to insulate the plug wire boots from heat.