"So I had an idea to reroute the fuel lines so that the fuel is always running through the heat exchange circuit. Full time winter and summer"

But that's how it came stock perhaps you are refering to removing the fuel thermostat which regulates the fuel temperature?

I ran a 617 for 22 years with outdoor temps into the -40's without a fuel heater or additives. The challenge isn't keeping it running but starting it can be. My current 602 has the fuel heater I rebuilt it 8 years ago just because but it likely would operate the same without the heater even in extreme cold.

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92 e300d2.5t
01 e320
05 cdi
85 chev c10