Ok, so let me see if I understand this correctly, you are assuming it was overheating because of steaming not from reading the gauge? I've reserected a number of engine that have sat for a long time, and especially outside, your going to get some stuff burning off the outside of the block when first fired up, so if you don't have a working temp gauge get it working or at least an IR thermometer to shoot at the thermostat housing otherwise you don't know what's really going on. I do not think that in 10 minutes of running the missing fan shroud would have made a difference, I'd actually be surprised if it could heat just the coolant in the block that fast let alone the radiator too. To run it a little and get it out of it's spot to be towed I wouldn't be worried about the shroud, but long term you should really get one. Look I know there are some folks out there running without a shroud and saying they haven't had a problem, but there is a reason they were installed from the factory. Low speed air flow will really suffer without it, if you have A/C it won't work well at idle, might overheat in traffic, especially on a hot day with load. Your fan may never see enough hot air from the radiator to kick the clutch on ect, ect I could go on. I would start by trying on from a 350 since that's what the truck came with.