Clean install! That US Cartool kit looks sweet. I used a 3" chromemoly driveshaft with 1350 Spicer joints from Strange Engineering.
Regarding driveshaft selection. Come to find out, mine is 52" long and has a critical speed of under 7000 rpm. My rev limiter cuts in at 6700 rpm, so in 3rd gear, I'm on the threshold of maxing out my driveshaft with my 3.91's. Once it hits the .67 to 1 ratio in 4th, it will easily exceed the critical speed.
I felt this as an intense vibration that built in intensity above 105 mph while I was doing some wide open runs down in Willow Springs a few weeks ago. This intense, high-resonant frequency vibration would kick in as I began to wind 4th gear up past 105mph through 130+ mph. I knew I had serious issues. With overdrive, my driveshaft can easily approach and exceed 9000 rpm on the top end. Catastrophic failure is only a matter of time away at these speeds.
Looks like I'll be ordering a carbon fiber driveshaft from QA1 this week. It's critical speed is somewhere around 14000 rpm.
Something to consider if you push your car hard.