Why not just drop the R65 speedo or tach into the hole and use the existing connections. If I swap out my headlight for one I am planning on doing away with the speedo all together and just mounting the tach in the headlight shell. A speedo is not a legal requirement (at least where I live) but you don't stand a chance of saying "honest officer I couldn't have been going that fast" without one.
I emailed the cafe site with fiberglass shells and got this back.
"For the 7" bucket, the gauge opening is 3.75", the 6" bucket has 3" opening. If you are using an aftermarket gauge, we could just leave the gauge hole uncut."