If this helps any, I have my 261 on an engine stand with nothing around it at all. The only thing that matters is the 1/2 inch or so clearance between the fan and the radiator. When I run it, I get a perfect 180 degrees max. (7 pound cap) So I feel you can eliminate any worries about sheet metal interference. You have a known good radiator right? The water pump is good? The thermostat isn't in backwards? Sorry, but trying for the simple.
