...even with poly mounts, you're still going to feel a **** load of vibes. Solid mounts are really nice but not that necessary unless you're going balls to the wall imho.
Also, the fact of the matter is you're putting in a LO5 - even if you put in rubber mounts you're going to feel a decent amount of vibrations.
...or you should just man up, get the solid mounts, and just get a nice cushion for your seat