From the research I did I thought the extra thickness fills out the remaining space below the top tank towards the engine. The pictures HandyAndy posted appear to agree with this. Do you remember where you bought yours? I'm not sure how you could move much forward with the top tank being so close to the top of the radiator support, but if you've got one that was a drop in replacement I'd love to find one.
My Desert cooler came from a 2 ton truck. I had to modify the top strap of the radiator support to make room for the thicker top tank. Everything else fit.
