I used a yellow fan with spacers on a tr3 and it worked well. I do wish I would have thought of using the nylon spacer idea that looks like it would hold at high rpm and still be easy to tool out the center. Nice job Dave. At one point, I thought I was going to use a tr4 hub and a yellow fan and try and stick that on the tr3 because the longer tr4 hub let the fan clear the brace and I would not have to use adapters (washers). However, that put the fan close to radiator, and it looked like I would have to slide the radiator forward by slotting the mounting holes. In the end, I went with the yellow fan on washers for the tr3 because I felt like I had some control over the gap by using washers as spacers. Having said all that a tr4 has different clearances, so I am not sure exactly what I would do there, perhaps one of those larger flex fans.
Steve