If you are going to upgrade the AM14U2 to use 6" wheels you need to purchase 160T belts if you don't want to modify the front and back rails. Im assuming that the HTD timing belts from VexPro and AndyMark are interchangeable but assumptions aren't always a good idea.
Here is the link to the AndyMark product.
Here is the link to the VexPro product.
Here is the link to the document that outlines belt sizes for different wheels sizes on the AM14U2.