Grand Prix (W-body) struts and springs are completely different (strut/spring mounting and spring rates).
Not to mention even if they did fit they'd make for an even rougher ride. The Bonneville will have a much better ride than a W-body if both of their suspensions are in the same condition. Even the FE1 suspension on even the most base Bonneville should have a smoother ride than a GTP with the premium suspension, when new. The GP will handle better, but as for road trips and regular day-to-day commuting the Bonneville takes the cake.
I suggest getting some new struts and get some FE2 or F41 springs from a Bonneville/Olds 88 or 98/Buick Park Avenue in a salvage yard.
For a good review on struts lookup "struts comparison" with Willwren as the auther -he has a seperate page on his own website with some valueable info on struts, springs, and other suspension items.