The dog bone helps locate the engine in the fore/aft direction, not laterally. The primary reason for the dog bone is to control the engine's torque induced moments.
There are four other engine and transmission mounts that hold the motor in place. One is located just aft and under the nose of the motor. There is a transmission mount on the front and rear of the transaxle. The fourth mount looks like a small shock absorber and is located just below the oil filter.
If you are getting lateral movement, one or more of these mounts needs to be replaced. BTW, when one mount breaks, the others work harder and tend to break. When two break, the others are really over loaded. This is a good reason to replace the dog bone before the excess fore/aft movement damages the other mounts (this is not unique to Fiero's).
Last november I replaced all the mounts on my V6 except the shock type mount (it's no longer available). I also installed the ultimate dog bone from FOCOA. When standing behind my car, you can rock it forward about 3 inches. Before I replace the mounts, the car would move something like 8-9 inches.
From: Lee Brown