Is your topsoil going to be dumped in piles in your yard, spread out in your yard, or you are going to place it with your FEL ? How level will it be when you start working it. If the soil is loose and fairly flat you might be better with a grader blade or landscape rake, they have a tendency to float better for a finish grade and not gouge(sp) as bad. However the more you drive on it the more compacted it gets and a box blade will dig in better and break it up better with the scarifiers(sp). I guess maybe a pulverizer might be your best bet. They are expensive, but maybe you could rent one. I have never used one, but hear they work good for making a finished yard, or even better yet a harley rake. Which is even more expensive, but probably the best. Never used one of those either.
With fresh topsoil that is loose I would rough grade it with FEL if it needed it, and then finish grade with grader blade or landscape rake. The grader blade can even be turned around backwards to float even better for a finish grade.
I always have a problem with my grader box going up and down and tilting as the tractor goes over uneven ground. That's why I like the floating ability of the grader blade. back and forth and criss cross a few times and it usually turns out pretty good pretty quick. Once again alot of this depends on the soil conditions you are working.
Good Luck !