I don't think the Remington factory clamp will work with any extension other than the factory version.
The factory extension has a groove the barrel clamp screw passes through and interlocks with the extension to prevent the clamp from shifting under recoil.
Without the groove in the tube, the factory clamp screw won't fit through.
However, Uncle Mike's does or DID make a close copy of the Remington clamp that will fit most any extension.
Email Uncle Mike's and ask if they still make it. Its not listed on the site, so you'll have to contact them.
Tank's Rifle Shop sells the same clamp:
http://www.tanksrifleshop.com/trs/cc...nkhdclamp.html