I'm not sure moving your coolant return tap will make much difference. If you move it to the downstream side of the aux pump (between aux pump and engine block) then wiring your aux pump to be ON won't do you any good. The return has to be on the intake side of the aux pump for it to be effective.
Aluminum tubing and 3/4" heater hose makes a good homemade HIH fuel line...as far as I know, greasecar still uses PEX tubing for their HIH. Frybrid uses aluminum tubing.
Your planned placement of the FPHE is good...first in line for the hot coolant, last in line for the VO (well, last right before the supply valve).
I see you're in Michigan, you may want to add a heated fuel pickup to your tank (such as a hotfox, it uses coolant to heat the fuel around the pickup). It doesn't need to heat your whole tank (better if it doesn't) just has to heat the area around the pickup so the solid VO melts enough to make it into the fuel line.
For my temperature display, I actually have a temperature switch like this:
http://www.dwyer-inst.com/Products/Product.cfm?Group_ID=149&sPageName=Ordering
It doesn't actually switch anyting on, just shows the temperature. I'd like to find another option, but I looked and looked when I was doing my conversion, and I couldn't find a regular automotive gauge with the proper range (and resolution). Maybe there's more out there at the VO vendors websites. I think I'll check right now, if I find something nifty, I'll report back
