You are going to have 2 problems with that setup. First is the shaft size. With careful use of a file you can get the shaft down to the 1/2" you need, OR you could see if you can get a 5/8 Jacobs chuck. The second problem is that the 1725 RPM is going to be a bit fast.for a flap sander. You probably will have better results if the RPM's were down to around 1000 - 1100. Other than those issues looks like you have a nice potential setup. Here is a link to a jacobs chuck on Ebay http://www.ebay.com/itm/Jacobs-No-36B-Drill-Chuck-3-16-to-3-4-Capacity-5-8-16-Mounting-Thread-Unused-/181189745657?pt=BI_Tool_Work_Holding&hash=item2a2fc01bf9
Hope this helps.
DW