If a new tractor is out of the question...
Depending on how you're going to use it that blade is probably on the big side as others have said. I have a RB1172 for my 3520 and it's about all the blade it can handle when I really start moving material.
Give it a try and see though. Heck, you all ready have it. Do you have hydraulic top N tilt on the tractor or the hydraulic offset on the blade? They go a long way towards helping to contral a rear blade.
I know what you mean about finding good (quality) used blades. I bought my RB in Maryland and had it shipped to Indiana and it was still cheaper than anything I had found around here.