Hi El Rojo,
I've home brew converted a 1984 6.9l IDI and an 01' PSD Excursion. Both have been converted pretty much the same way. Couple of hints:
1. the oem fuel selector valve on your truck is pretty fragile when handling WVO. They're know to fail. They're known commonly as Pollack valves.
2. I would replace the 6 port Pollack with 2 3-port Hydraforce valves or some other equivalent as they are proven to work fairly well at the temps we're talking about.
3. There are different schools of thought on heating the tank itself. I subscribe to heating just the portion of the tank nearest to the fuel pickup - in the case of an Artic Fox it acts as the pickup as well as the heater I have one in my rear 45 gallon on my f-250. Many folks mount them horizontally with the idea that they would be more effective keeping the whole bottom of the tank fluid. I mounted my 14" with extension vertically and it works a treat. I do not burn PHO and my winter WVO is skimmed off the tops of my containers - only the most fluid of the fluid oil makes it into my tank Nov-March.
4. The replacement fuel selector valves should be mounted in the engine bay which will require making some space and a mount for them. Most folks recommend running completely parallel fuel systems, so you can switch in the case of failure.
5. On my IDI I deadheaded my mechanical pump and filled it with some light oil to keep it well lubed as a backup. And placed two electric diesel pumps along the frame rail - one pushes WVO and the other #2.
6. I would suggest you look into making a hose in hose "HIH" coolant jacketed fuel line for your wvo to go from your wvo frame rail mounted filter to the engine bay.
7. In the engine bay you should loop your return line using your selenoid valves so that you are always returning warm wvo into the loop which will save you some btu's.
8. I would suggest the use of a 26 plate FPHE in the engine bay as well - and in my case I also have VegTherm Mega 1500w 30a electric fuel heaters inside the loop. The 97' is HEUI I believe so you need not worry about thermally shocking your injector pump. many folks with IP's run both diesel and wvo through their FPHE's that way the fuel when switching is always within a few degrees presumably saving you the cost and aggravation of a shattered IP shaft.
I learned all of this after much, much reading, practical experience and toil, I thought I'd share it with you even if you didn't ask for it
To answer your question it would seem to me, and my OA seems to prove it out that somewhere in the 160F pre fuel rail temps work well and are easily achievable. I have a thermocouple from a coolant temp gauge in line right before my fuel enters the rail to tell temps.
Hope this helps.