Adding 20 ft of hose will slow the flow of the coolant.

Are you using the hose from the in-cab heater? or the main 2" hose?

If you are concerned, heres some vague instructions.

Make a 3"long copper pipe, with flanged ends that will fit into rubber coolant hose.
Make 2 holes in pipe. Add 2 nipples for small hose.
Splice this unit into your coolant hose, and use hose clamps.
This small hose, add a small 12v water pump, and use it to run hose to your wvo heater.
You dont need a huge line for heating the wvo, a 2-4 gpm pump should work awesome.
If you added a heater in there somewhere, you could heat the wvo before even starting the truck.

When you read "solenoid valve", what is meant, is a valve, that instead of turning on/off by hand, is controlled by electricity. Usually 2 wires, and it either stays closed or stays open with a spring, and then acts the other way when you give er 12v.

Purging is stopping the wvo, and going back to diesel, tu burn the wvo out of the heads and injectors for an easy start.
I.E. CLOSING the wvo valve, OPENING the diesel valve, and STARTING the diesel pump, SHUTTING OFF the wvo pump.

You want to wire in a switch, looking like a front/rear fuel tank switch.

position 1 will be say diesel, and it will energise the stock diesel pump. It will also energise the DIESEL fuel solenoid, opening it.

position 2 will be wvo, and it will energise the wvo pump, and wvo solenoid.
Keep in mind that the solenoids close as soon as you take away the power, (or some stay open unless you energise them in which case your wiring to the switch would be backwards)

I'm not 100% sure which wires to the fuel pump you use for turning on/off, but i'm sure someone here does...

Once you install this switch, you will leave it on "diesel" for starting , and shutting down, and once the motor is running, and the wvo is hot enough, you switch it to wvo.

"T" the power wire from the existing fuel pump to power your wvo pump, that way the truck can turn it off in an emergency.....or it it overheats it will blow the fuse instead of frying the pump....

DONT USE COPPER IN CONTACT WITH WVO
it reacts.

If you get a decent plastic tank, or transform an old slip-tank, that should work good. By transform, of course i mean add the fuel supply, heat exchanger(s) and you also should put a thermometer probe near the fuel pump inlet, to monitor the oil temp.

If it gets cold where you are, i'd put extruded polystyrene around the whole thing, to keep it hot better.