I'm in the process of removing both the Schumacher Polymounts and poly transmount I just put in my '68 during my Magnum upgrade. The vibration was just too much at idle and everyday driving. As stated, they were better once rolling, but even then, the added vibration was an issue. There are many post on the internet advising against using stock mounts alone. They just aren't as good as they used to be, and they were regularly breaking under performance use.
My current small block motor puts out about 480 hp, so I knew it would likely break a stock style rubber mount. I didn't want to mess with chaining it or adding torque straps, so I ordered up a set of Mitymounts. http://www.mitymounts.com/chrysler.htm
I have them in my shaker hooded '70 E-Body. That motor puts out 710 hp and nearly that much torque with a 5 speed. They perform flawlessly. There is enough give in these mounts to allow my shaker to do its thing, but they solidly lock under power to keep the engine from destroying my hood set-up.
They are as easy to install as a stock mount and I highly recommend them. I am using a rubber and steel transmission mount from Trans Dapt Performance. It is stronger than a stock mount, but still gives the benefit of not passing vibrations through to the chassis.
Here's the link to how they work. http://www.mitymounts.com/howtheywork.htm
