Thanks everyone, always great advice here. I kept the radiator hooked up to the cooling system, but tilted one corner up until it was at the same level as the system cap mounted above the thermostat (see pic). I added fluid until it was flowing out of the bleeder valve on the radiator, closed it all up and ran the engine for a bit, let it cool and checked the coolant level again. It looks like I have all the air out of the system now and on several test runs the engine stayed at normal temp the entire time. The large truck radiator and twin 14" fans seem to be doing the job of cooling, but I guess my question now is will air accumulate in the radiator over time, creating the same situation I just fixed. The filler cap is the highest point in the system and I added a bleeder port to the top of the radiator end cap to bleed any accumulated air. If air is going to be problem, I will have to look at mounting the radiator upright somewhere else. I like the low profile look of where it's at but I need max cooling capacity. This is going to be a parade vehicle, so it is going to be idling on summer days. My other thought was to add a small vertical radiator with an electric fan at the front of the engine and run the heater hose ports to that secondary radiator.
It was great getting this silly thing running. It rides a little stiff, but it is very stable for something that looks top-heavy: