If you want a stiffer rear end that still is squirelly and still has bump steer, poly and sway bar the early chassis cradle.
If you want a stiffer, squirelly rear end with wierd bump steer characteristics, put a sway bar, poly bushings, and a bump steer kit under an early chassis. The bump steer kit smooths the bump steer for only PART of the suspension travel, but not all of it. So on a bumpy road that causes the suspension to travel more than a couple inches, bump steer is limited a little, really there, limited, there, etc. Really a handful!!
Of course, you could get the suspension so stiff that the car will never move more than an inch or so, but how comfortable is that on the streets?
If you want minimal bump steer, an excellent handling Fiero with a good ride then do an '88 cradle swap.
My last 5 Fieros have been done this way, both racecars and daily drivers. The 4.9 with 5 speed on an '88 cradle with poly link bushings is one of the best handling/driving Fieros I have owned (including the Q4 racecar that I had over $25K in). If you make it to Ed's swap meet, look me up. I will let you take it for a spin.
G