Rebuilt heads won't help with a broken block.
You may want to speak with VAC Motorsports. I know they can sleeve the blocks. Maybe sleeve a 4.4 block to the 4.6 bore and rebuild from there?
You may check with these guys
http://http://www.data.bmrparts.com/bmwparts/BMW_ENGINE_ASSEMBLY/BMW_11007515721_4.6L_used_long_block.html

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