Several Bonneville guys have done 7 main GMC's before for the cranks and cams. It requires machining and welding new structures into the block. Just have LSM make you a billet block. It'll end up costing about the same in the end as converting a 4 main to a 7 main.
Your bore size will be limited to core shift and other dynamics relative to the build, such as compression and cylinder wall loading from rod length. You may have to sonic check 10 blocks to find one that can be bored that large. You should also prep 2 or 3 blocks to have something to fall back on when you have an unexpected engine failure for some reason. You'll also need to make the cam 7 mains as well. We regularly broke roller cams in our Comp Eliminator engines with 4 journals. Even making the journals larger so you can make the base circle and cam barrel larger wont last if you still retain just 4 bearing journals, regardless of the material the billet is made from.
