10 awg seems reasonable for the thickness of the wires on my 84 & 85 300D glow-plug wires, but 12 awg would probably work too. Use wire rated for oil immersion. As to length, each one is a different length. Can't you just cut them as you install? You could grab a harness from a junkyard as a guide. For the plastic sheath they run in, search "PVC 105" from "SPC Technology" or "Alpha" on ebay. You need larger (~1/2"D), but that will get you in the neighborhood. I use it on when rewiring my 60's cars, or you could use "split loom", but I think it is ugly and hard to keep clean.

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1984 & 1985 CA 300D's
1964 & 65 Mopar's - Valiant, Dart, Newport
1996 & 2002 Chrysler minivans