Someone correct me if I'm wrong, but I believe the 46GT may fit but you won't be able to turn all the way. The machine it would have come off of has swept forward front axles giving more clearance for the deck. Also, when/if you do get it fit under your 1811 it won't life very high at all. Good luck with whatever you decide!
As far as a poor cut quality with the 44C, are you blades sharpened correctly? Maybe you're going too fast? Are you running your engine at WOT? All can greatly effect cut quality.