My advice would be to go with the 659 as it uses the M16 mount.  The M16 mount has two sets of screws (a longer set & shorter set) and you can use it on the front sight housing or use it in front of the front sight housing.  Here's an illustration of the 659, 660 & 674:

[img]http://img.villagephotos.com/p/2003-8/315352/bARmnts.jpg[/img]

Here's an M16 mount (still using the longer set) positioned directly to the barrel:

[img]http://img.villagephotos.com/p/2003-8/315352/Im001274.jpg[/img]