[Christ]

Wow, I somehow thought that you only had the one truck, and were converting it on a time budget. Woe is me!

I think the two spots that lose the most heat in the engine are the oil pan and the valve cover. If you can insulate either of those, you'll likely keep more heat in the engine. The oil pan shouldn't be too difficult, if you can find another OEM one, gut the windage tray from it, and replace a couple of your oil pan bolts with studs that are long enough to hang the second one on as well. You might have to hammer the second one out a little bit for it to fit nicely, but that will allow you to use airspace or some decent insulation to keep heat in the oil and protect against air movement releasing heat from the oil through the heat sink effect.

The valve cover... you've got me on that one. No idea. The heavy iron block and aluminum head probably don't lose too much heat comparably.

The added benefit to using the second oil pan is that you can add a heater right there in the pan.. I just suggested this to Orange4boy in his Prius thread. That pipe-tape heater you used on the Electro-Metro's batteries just might do it for you, if it's a decent heating element. It's nice to heat the coolant, but oil flows through the entire engine, and touches places that the water never sees.