You will get many responses on this question. Several folks have used spacers. Depending on your rims and offset etc., spacers may work well and help keep the rim centered over the bearings (again, depending on rim offset and depth etc.). Some folks also use spacers regardless of the centering. this can lead to bearings wearing out prematurely but I haven't seen any debates on failures yet.
In my opinion, creating widened Fiero suspension would be the way to go and I have done it for my Ferrari 355 build. There are options out there such as extending and reinforcing the Fiero control arms, building tube extended control arms, buying Held/Arrault (not sure what the company's name is now as it has changed hands a few times since Dave Held sold it) extended control arms or whole setup including cross member.
There is also a thread over on Pennock's under the Construction zone where Bloozberry (userID) is making a custom rear setup for an '88 Fiero cradle that is very interesting and detailed. he has gone through significant design and testing to obtain the same or better geometry and is now building everything. It is a spectacular build thread and the suspension starts closer to the end as he also showed his N* rebuild at the beginning.
If you have a pre '88 Fiero, the rear suspension has bump steer and can be removed a couple ways with the Held bump steer remover control arms or moving the trailing arm mount out to over the control arm pivots. I did the pivots and it removed 95%+ of the bump steer from my setup without the expense of the bump steer control arms. '88's are pretty good as is.
sway bars etc. can be purchased for pre88 rear suspension. or you can replace the front one with a larger aftermarket one and use the front sway bar on the rear cradle.
There are also many upgrades to brakes and hubs depending on what you are doing for rims and final look.
Your best Fiero resource is really the fiero.nl site (Pennock's) as the folks there have done pretty much everything you can and want to to a Fiero.......
There is another Fiero site as well with good info but the folks on that one are pretty crude and seem to spend more time swearing than it suits for me. I think it is called Realfierotech.com?
Good luck
Don