I realized that I could go back and start over, or change to a smaller wheel. I contacted Udo and he informed me that he had use a 6" x 2" Matco wheel on his Super HP-18.
When I contacted Matco I was informed that they had recently produced a pneumatic version of the 6" x 2" wheel. (Matco part number WHLTW35) I purchased the wheel with and a spare tire and tube.
The picture below shows the size difference between the Matco 6" x 2" wheel and 210mm x 65mm (8.27" x 2.56") standard sailplane tail wheel.