I just went through something somewhat like what you are describing just last week. After putting up an attached barn we needed to regrade to get a reasonable driveway and slope everything away from the house rather than into it. We were taking on no more than 1/4 acre so it's quite different in size, but it had the added charm of being almost entirely ledge, and where it wasn't ledge it was evidence of why New England is littered with stone walls. Ours grew a bunch last week!
Anyway, from my limited experience so far, I'd suggest considering the soil makeup before selecting your implement. Breaking up the ground is key to being able to subsequently move it somewhere else. The scaifiers of a box blade will work well in almost anything up to soil heavily laden with rock or ledge. The worst parts that we faced I actually broke out the backhoe and broke it all up that way. A tiller will work well in clear soil, but if there are rocks I think a box blade with scarifiers will be better.
As for actually moving the soil, I ended up using the loader almost exclusively. Why not the box blade? Pretty simple - the loader holds more material than the blade, and the blade is really meant to even a grade, not move stuff from one place to another. It's meant to be filled with soil and cut where it's high and fill where it's low, so it smooths localized irregularities, but is not so great at taking a hill over there and using it to fill a valley 50+ feet away. I found scoop-drive-dump to be more effective. I also find the box scraper to not work so well with rocky soil. The box blade and grader blade are great for finishing work, but not for rough work.
Net result is that we were able to do a nice job and my wife now really appreciates the tractor and even insisted on using it for a while - something I never thought I would see - but I'm getting side tracked.... What I was getting to is that even our relatively modest job took us 3 days and it sounds like you are facing much more.
Here's another thought you might consider, if you haven't already - what about putting in a french drain the length of the low spot and empty it into the stream? Basically drain the wet area rather than backfill it. This of course assumes that the low area is still above the water level in the stream. Best part is that you'd have the justification to get that Backhoe!
<P ID="edit"><FONT SIZE=-1>Edited by hayden on 07/13/01 09:33 PM (server time).</FONT></P>