Do NEVER use weld together!!!
If the boneassignements are correct:
Select again 2 vertices(one showing) and use snap together from the vertex menu
Repeat these steps for the whole seam.
this is normally the first step to fix it but i thought you already had done that.
When you snapped all pairs of vertices you then also need to alligne the normals:
Select the whole seam between the feet and the body and select alligne normals from the vertex menu.
If the plugin is not there get it here:
http://www.modthesims.info/showthread.php?t=139859