Sendero,
A box blade has a few good uses. It's an affordable tool for maintaining a gravel driveway, it will smooth out some small rough spots in a field and it makes a good place to store things on top of. Otherwise, it's not really worth the effort.
You said you have a FEL. Dig the hole with that. If it's a little wide, that's fine. Your FEL will dig much better then a box blade ever will. If you need teeth on it, that's something you'll always appreciate.
The box blade will move a little dirt around, but not nearly as well as your front bucket. Remember, the box blade is for "SMOOTHING" soil and gravel.
After you put the culvert in, then it's real easy to cover and spread with your fell. I think the FEL is a great spreader for dirt. It shapes the contour just as you want it and gives you a little compaction. Driving over the loose soil with a load of dirt in your bucket also helps get compaction.
I like to back drag my piles real thin with the back part of the bucket and use the bottom part for shaping. By the time I'm done, there is no settling and I get the results I'm looking for.
This is a fun, simple easy project that shouldn't cost you anything more then the culvert.
A 12 inch culvert here in East Texas will run around $110 for a plactic one with a smooth inside. I like them allot better then metal ones. Metal will rust in time, they get dented and the ends usually start to look a little too rustic for me after a few years. Plastic will look just like the day you bought it 20 years from now.
Eddie