LRRPF52,
It is my understanding that this is exactly what the Anvil is... You do not have to worry about hoop stress or breaking bolts, as this is an AR-10 width upper, extension and barrel. Just that the upper is shortened to handle 2.3" coal instead of 2.8". All the meat is still there. Kind of the opposite of the DPMS GII.
Does it not follow that the lower receiver is as wide as as an AR-10 as well? Look at the picture you posted. The receivers appear to match up perfectly. So... it may have enough material that the magazine well could be further opened.
As it is... this is an attractive combination. No fear of taxing the limits of an ar with the Grendel. I would like to see what performance the Grendel could achieve in this rifle. A twenty two inch or longer barrel would be a nice option.
Opening the magazine well to accept six8 magazines might be possible. Removing a section of a .308 magazine and tig welding it back together might also be an option. Just picturing the lower opened for such a modified magazine and the upper chambered in .30br. or 6mm br. Bolt hold open might not work, but, could rounds as long as this or .22-250 be made to fit?
DB