you can cut the coupling with a sharp blade or tin snips to meet your needs. Maybe crutchfeild will throw in some extra rubber spacers to fill the gap. 1 5/8 seem had to find and the company i got mine from provide the 1/8" additional spacers. yes they do seem pricey.
What tower speakers did you get.? Btw the tower looks great
I got the Polk Bullet speakers, and as for the clamps I had some custom made to fit on the top support instead of tubing. They still cost alot but I like the look. I'll send photos later. Oh and I didn't need rubber spacers.