540*6/9=360 rpm that the driven shaft is traveling. You want a ratio that keeps that speed on the driven shaft.

That sounds low to me. I think you may have your pulleys mixed up. I got a feeling the 9" is on the pto shaft (driver) and the 6" is on the driven shaft that the spinners are on. Better make sure because it does make a difference.

Going on with the example assuming you are correct with which is the driver and the driven then you want to keep the driven shaft at 360 rpm.

If you want the driven at 360 and the driver at 1000 then 1000/360=2.78 or in other words you want the driven to be 2.78 times the size of the driver. So with a 4" driver you would need 4*2.78=11.1 or about a 11" driven pulley. 1000*4/11=363

Like I say, I think you have your driver and driven mixed up. I'll go with an example assuming you have a 9" on the driver (pto) and a 6" on the driven. In this case your driven shaft is 540*9/6=810 rpm driven shaft. This is closer to what our buggy is. In this case 1000/810=1.23 ratio so if you changed the 6" from the driven to the driver (the pto shaft) 6*1.26=7.56 or about a 7.5" pulley on the driven shaft. 1000*6/7.5=800. So to recap, switch the 6" pulley to the pto shaft and put a 7.5" on the driven. Or you could do a 5" on the pto and a 6" on the driven 1000*5/6=833. Or 7" on pto and 9" on driven 1000*7.5/9=833. If they are the tin pulleys there is a fair chance you will trash the one you take off.

Or what I would do is put 6" pulleys on both driver and driven (or 7" on both or 8" on both or 9" on both) then lower the engine rpm to save fuel. With a desired rpm of 810 rpm with a 1-1 ratio you would need to run your tractor pto at 810 rpm to get 810 rpm on the driven. If your tractor has a digital tach that displays pto rpm's you are set. Or calculated, if your tractor delivers 1000 pto rpm at 2200 engine rpm to get 810 it would be 2200/1000*810= 1782 engine rpm or rounded off 1800 engine rpm. If you run a big tractor on a fertilizer spreader it will save a bunch of fuel to run at a lower engine rpm and most engines develop maximum torque at about this speed so if you get in a mud hole you should still have maximum umph (its a technical term ) to get through it.

John