I've got to agree with Henro's recommendations on equipment. I would also secong the recommendation for a toothbar for your FEL. You might get by without the boxblade in some places, but you'll find it a real timesave overall, and indespensable on side hills: A box blade (or a rear blade) will let you change the angle on a side hill (i.e. leverl the trail left-to-right) much more easily than doing it with the Loader, since you can adjust the tilt on a boxblade or rear blade.
I'd recommend reading up a bit on some of the threads here on dirt/gravel roads or driveways (there are a few links to good info in some of these threads as well). A lot of this will probably be overkill. You don't need to know a lot about geotextile fabric or type or gravel surfaces for a typical "trail", but some of the stuff on erosion and drainage will be good reading.
I'd recommend against trying to build a trail straight up slopes as steep as you mentioned. Yes, it's easier & safer to drive your tractor straight up a 25% grade than to drive across it. But you are asking for serious erosion problems with a trail like that. If you do switchbacks, or follow terrain contours when possible, you can cut the trail level with your FEL, backblade (or backhoe, if needed) when on a side hill. This will give you a nice, safe trail, and be easier to control water flow.
John Mc