Hey Blackfoot Tucker, thanks for the advice and taking interest. I will start by apologizing for the long reply, cooling this "Monster snowcat" has been an odyssey. To answer your question we are using a champion radiator with a pair of fans. The guys at Becool laughed at me and said that my radiator was junk but others have said that it's OK. Luckily we haven't made an engine cover, because it gets hot, so the only limit to what size and brand radiator I can use is my wallet. I will definitely give Ron Davis a call this summer and look into a giant radiator with super powerful fans. It's always funny to put racing parts on my cat.
The main issue is that the radiator is in the back and doesn't get a lot of air. My foot is another issue, it runs way better at lower speed and rpm. And in all fairness we do ask it to work pretty hard. Also as you probably know the sensor on an LS motor is in a goofy location and reads a lot of exhaust heat. I have gotten suggestions as to getting a true water temp but that is a double edged sword. Currently we get the worst case scenario from the temp Guage and that helps keep the head gasket safe and gives you a better idea how hot the head is. If we get a true water temp I will probably feel comfortable driving it further risking the head gasket.
I have had a lot of success with cheap solutions and I have one more to try before I replace the radiator. Someone in McCall suggested a Coolant additive called water wetter. And we seem to be pretty close to cooling it properly so I want to see if it will "get me there".
Cheap solutions that worked were, tightening the sending unit, switching fan direction, raising the elevation of the fill, and utilizing the steam lines.
Sean,
We spent a lot of time trying to make the Griffin radiator work. The engine's computer uses a sensor in the right cylinder head. The in-dash temperature gauge is not compatible with that sensor, so we added a second sensor in the left cylinder head. The temperature read by the computer was quite a bit less than what the in-dash gauge showed. We drained the coolant, swapped sensor locations and determined for some reason the left head runs hotter. We then drilled and tapped the thermostat elbow area for the in-dash gauge sensor.
I considered using Water Wetter, but saw mixed reviews. I'm thinking about using Evans Waterless Coolant, but you must use it at 100% concentration, and it's almost $50/gallon. I think I'd have to spend almost $300...just on coolant. That's an "ouch".
The electric fans are controlled by the engine computer. The guy who re-flashed the computer for me programmed both fans to come on at 204º and off at 196º (IIRC). I'm going to have him change that with one fan coming on at 184º and off at 176º, and the other at 204º and 196º. That way one fan will be running basically continuously and the other on an as-needed basis.
We also added a Derale remote engine oil cooler that is thermostatically controller and has its own electric fan. That's mounted to the frame under the rear floor and right behind an identical unit that's used to cool the transmission.
The fan and shroud setup from Ron Davis is well designed and beautifully constructed. There is quite a disparity in fans as well. Some flow a lot more air than others.
The radiator in the Tucker is right in front, but at Tucker speeds there isn't much speed-induced airflow. The fans do the work.
Sno-Drifter's point is well made, and completely on point. My Tucker's factory hood was a butterfly style hood with a hinge in the center. I hated it. Both earlier and later vintage Tuckers came with a tilting hood setup. Unfortunately I haven't found a snowcat salvage yard with a bunch of Tucker tilting hoods, so I went about having one made. (It's a long, ugly story.) The factory hood has 12" louvers in it, and they're made with a louver punch. Nobody in SLC has a 12" louver punch. The best we could do was smaller louvers and more of them.
But under hood heat is a problem. The exhaust manifolds and pipes radiate a huge amount of heat. Currently the machine is completely disassembled, but when we put it back together we will wrap the exhaust pipes with a ceramic exhaust wrap which we hope will cut down a lot of the under hood heat.
So the local area is closed, I just bought new skis yesterday to replace my 'daily drivers', and I'm sittin' here hoping to live vicariously. 
