Here's the best thing I've found so far:

According to Dr. Christopher Jacobs (of Omni-Pak and Pro-Street ignitions from Jacobs Electronics), indexing sparkplugs 'never hurts and sometimes helps'. Indexing is especially useful if the combustion chamber has a 'squish' area where the piston almost touches the head. This squish area 'squirts' a high speed flow of mixture toward the spark plug. The Dodge Magnum heads do have a squish area (also sometimes called a quench area). The spark plug is at the 'outside' (toward the fender) of the head and the spark plug hole is angled toward the exhaust valve. The intake and exhaust valves are in the middle of the chamber. The squish area is 'inside' toward the intake manifold. The combustion chamber is not round. It is 'kidney' shaped with the tip of the indented 'v' of the kidney pointing toward the spark plug. The difference between 'round' and 'kidney' is the 'squish area' where the piston top almost hits the head at top dead center.
For indexing, Christopher Jacobs recommends that you mark the insulator of the sparkplug with a black magic marker (not a pencil as graphite conducts electricity). Draw a line on the insulator on the side where the gap is open - opposite the side where the ground electrode is. Seems like you could also scratch/grind a mark on the top metal electrical terminal so that you could see & feel it. To align the spark plug's open gap toward the quench area, you would need to have the marked line on the 'top' side - away from the fender on that side of the truck. If you are looking down on the installed sparkplug, this means the mark should lie at either 11 o'clock (if the plug went in angling slightly toward the front of the truck) or at 1 o'clock (if the plug went in angling slightly toward the rear of the truck).
In practical terms, any o'clock position between 10 o'clock and 2 o'clock would be in the ballpark. This would also mean that if the open gap's mark lay in positions between 4 o'clock and 8 o'clock it might be bad - the ground electrode would stand like a fence post partially blocking the spark gap from the squish area.
Jacobs sells washers to go onto the sparkplug's seat to cause the plug to line up this way. You can also file off a little bit at a time of the start of the first thread to cause the sparkplug to end up turned a little. If you have a lathe you could turn off just a bit of the seat.