Your math is right as far as it goes, but 1000 RPM is actually below factory idle for these machines if I remember correctly. Your engine should be running at max speed,3000 to 3600 RPM when you are tilling. You need to get more speed reduction. Also, I think I saw somewhere that for power transmission, bigger pulleys are better. The belt has a longer area to grip while going around the pulley and the leverage is better. I would strongly suggest that you use the stock pulley under the tractor and put a larger pulley on the outside for the engine to drive. I would tend to keep the small pulleys at the 90 to 100 mm size if that is the size of the original engine pulleys, and do all my speed reduction by driving 150-250 mm pulleys.
Keep in mind that as you lower your RPM that the torque available will go up. So the engine at 3600 RPM may be able to put most of that power through a drive belt with 4 inch pulleys. However, if the implement you are driving is going more slowly than the engine, the torque will be much higher. I think the engine on your tractor will put out in excess of 15 Nm of force at peak power. If you have a 10:1 reduction in your gearing, your tiller will still be turning at 360 RPM, but will have 150 Nm of force available.