Why do you want urethane mounts? You're going to get a lot more vibration and noise, and no performance advantage. Urethane engine mounts are for racers with mad power and transbrakes that launch hard- they control engine wind-up and shave off a few hundredths. Unless you're running nitromethane in your Six, you don't really need them.
Urethane mounts/bushings on daily driven cars is a bad idea if you like a smooth/quiet ride. Urethane parts only show a difference when you're pushing your car to its absolute limits.
Good luck finding rubber aftermarket parts though. I've been looking around online for rubber spring isolators and control arm bushings and haven't found ****
I figured if I was going to spend the money on new mounts, I might as well upgrade. So, are they available? Or are the stock mounts that went 117k so far OK? Like to have options.