A narrow ditch is fine, until the sides collapse or it needs cleaning. I had a similar problem, and constructed a 5' swale by digging a trench with the middle buster, and then dropping one set of wheels (one side of tractor) into the trench, putting my tractor in FWD, locking the rear differential (straight trench), and carving out both sides of the swale (one side on first pass, turn tractor around, and then the other side). An angled box blade will certainly do the same thing, albeit with more passes. Not sure about the scrape blade - depends on how dense / heavy the soil is, rocks, etc. Anyhow, I planted grass in the swale, and can mow it easily. The aforementioned was not my idea - someone else on the forum - without which I'd probably mess up most projects I attempt.