[Hufman]

Copper plugs provide no benefit over iridium plugs in dodge darts. All spark plugs have copper cores, you're paying for the electrode tip metal. Copper plugs have a copper core with nickel tip and iridium plugs have a copper core with an iridium tip. Both of which doesn't matter as both have internal 5k ohm ceramic resistors (to limit RFI and damage from capacitance after-fire). Copper-nickel plugs are 2x less conductive than iridium, 2x softer, foul easier, and fire less consistently. They aren't even cheaper, at half the price, but need to be replaced 10x for every one set of iridium plugs.

 

[Hufman]

Yeah, I replaced them all. The bottom mount wears out and then the sides mounts flex a bunch of it wears them out next. If I had to do it again, I'd get the Deyeme Lower Flex mount and OEM for both side mounts.

That was my thought process as well. I filled the bottom mount with polyurethane as soon as I could, and the original mounts are still going strong at 80,000 miles.
My only worry is the extra strain that might be transferred into the small rubber bushing, where the bolt mounts. Assuming the polyethene filled mount will flex less, the rubber at the end might take more strain and fail sooner, knocking and swinging on its axis. I haven't looked into it that much but when/if that part fails, hopefully I can just make it a solid mount at that bolt end.

 

[Hufman]

Honestly did not notice a difference tightening up the bottom mount. Both the upper mounts hold the weight, I assume filling those with some high derometer poly would increase vibrations. However if you stick to a 40 or 50 derometer, it should be even softer.