It depends which chopper you have now. I assume you have the old tri-rotor style with the small tailboard fins. In that case you cannot just change to the paddled knives jdmanitoba is talking about. Those will only fit on the newer fine cut rotors. The cheapest way to spread 36' is to install a Redekop MAV rotor upgrade. It has fan paddles on the outer ends of the rotor to get max spread, but uses straight blades on the rest of the rotor so power consumption is hardly any more. It comes with pulleys and a double rib belt for the rear drive that speeds it up about 300 rpm to approx 2400. An additional tailboard fin can be added to ensure a full 36' spread. If you want a finer chop an extra fine cut kit is available with a new knifebar and floor upgrade.
I assume by your user name you're from North Dakota. Not sure how close you are to me, but I'm a Redekop dealer and I'll give you a good price on a chopper. Call my cell anytime 204 371 9928. Chris.