I am doing a tune up on my 3.9 and I got the Autolite 5224 spark plugs from Rock Auto, but I have been reading that the Autolite 3923s are better for the Magnum engines, should I get those instead? The ones I got where only $7 for all of them so it's not a big deal.

 

If you live in a warmer climate or have slight pinging, yes. Otherwise the 5224s should be fine. The difference is the 3923s don't have the extended electrode, and they're a heat range cooler. They don't cost much, so maybe grab them anyway and test, if you don't like them, you already have a set for next tune up.