I'd recommend the 12V plugs, they glow hotter and are less likely to burn up on a 12V electrical system. I usually hold mine for about 5 ish seconds, but it depends on how cold it is. I'll also usually hit them a few times after it has started on a really cold day just to help things warm up. Do you still have the factory fuel filter/water separator mounted on the firewall? And where is your lift pump located?