The 4.2 is weird like that...see, one side of the ignition fires on the positive side, the other side fires on the negative side. When the 4.2 first came out, Ford specified two different spark plugs for that engine, so you'd have different plugs on opposing cylinders. They've since updated the plug to where there's only one plug, and it'll work for either side of the ignition. I'd split hairs and lean towards .050. Make sure those Autolites are like the latest Motorcraft plugs, in that it doesn't matter where you install them, that they can fire on either side of the ignition.