If you are getting some type of vibration at the pedal, I personally doubt attempting to isolate the mouting in any way is going to achieve great success. First, the pedal has to remain in a fairly rigid mount. To do this you have to maintain contact with the studs, no matter what. Even if you place dampening material between the back of the assembly and the firewall the studs will still transfer whatever vibrations might be present. Second, when you place it against something less rigid, it will have a tendancy to carry the vibration more.
IMO, I think you need to find the source of the vibration. Granted a 5cyl does not run as smoothly as an 8cyl, but it is far reach to accept that engine vibration (by design) easily makes it to the accelerator. Since it is an electronic it is not transfering it through a cable, the motor mounts and the body mounts would further isolate the firewall from direct vibrations. It could be resonation there but one would assume that would be rather low frequency and you would "feel" it in other places.
Try to think of the cause. Does it happen more at idle or at average RPMs? If you rest your other foot on the brake pedal can you feel it there? Does it vibrate at the same rate that you hear the exhaust note?
I would just fear that you might end up with a worse situation without exploring possible causes for it.
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