418 will work, the one piece (and probably two-piece) Durasight base will work. I found some S418 Weaver Grand Slams at Numrich but they sold out of them after I bought my second set.

Any bases made for CVA round barrel models should work. http://www.midwayusa.com/eproductpage.exe/showproduct?saleitemid=996237

http://kinneysshootingsupply.com/warne-m980980m-mount-p-334.html ? I'm not familiar with that dealer.