The 13/16 hex of the old LA head plug won't fit a magnum head. The thread and reach are the same but there is not enough room to fit the plug and socket down into the head recess. The magnum head plug has a 5/8" hex and is actually a great choice for LA motors with headers, the smaller 5/8" hex socket will fit between the tube and the head on cylinders 5 and 7.
THe magnum engine (3.6,5.2,5.9) spec'd in Autolite numbers a 3924 from 92 to 96 and a 5224 from 97 up. I use 3924's in my engine (the nose projection is slightly less on the 3924). If you feel the need for a colder heat range there is a 3923 readily available.