Given the report that the truck was grumpy to start at 40* F and then seemed to run normally once it lit, I'd focus on the GP system.
On a semi-related note, avoid the temptation to energize the glow plugs while using a battery charger as not all GP's play well at the higher voltage level.
If you know somebody with an IR camera, a quick test to isloate which GP's are working is to use the camera after a few cycles. The ones that are working should show a small rise in temperature. Naturally, this presumes a completely cold engine bay and I would not pre-heat with the block heater. Goal here is to find which items in the GP system need attention (and not start the motor).
Aside, my take on the block heater is that something is not up to par. Reasoning is that I saw about a 30 - 40* F rise in temps after about 3 hours of heat time. So, perhaps the generator is not putting out or the heater element is due for replacement.
Mods: might consider breaking this thread at the appropriate point and moving it to the 6.5 section.