Dunno about long action KRGW3 chassis, but:
Yes, there was an M2008 supermag action for 338LM.
No, the M2008s are no longer available new from BO.
The M2008 was superceeded by the M2013 which is nearly the same thing with minor alterations including a lighter bolt lift, faster lock time, and some "lightening cuts" that are pretty much aesthetic only. The bolt release was altered as well, and is more typical of the ones on other Rem700 clone actions. The 2013 requires the same stock inlet as the 2008, and is still available in long action with 338LM bolt.
Hope that helps.