Does anyone know if the steering knuckle off a 98 grand prix will fit in back of the fiero without any heavy modification? It looks offal close to the fieros just looking at pictures.

probably not, the GP's use a much more upright strut than a Fiero.

MWhite did it for his LS4 swap that reused the grandprix aluminum cradle. In this picture, you can see the mis-match between the upper bolt hole location on the strut and the grandprix upright. I assume he welded a tab to the strut flange to pickup the now bolt hole location.