It is being filled due to the large over bore to strengthen the cylinder walls. The goal for the engine build is high performance street, not for track.
If its for the street, no way I'd fill the block.
Why not just sleeve the cylinders? Or use a different block. All that said I know it been said before, why not use an LA 360 roller block? You'd be miles ahead with a roller and 360s are easy to find plus cheaper.