Because of the bigger bore of the 261" the casting has 'siamesed' cylinders on 12,34,and 56. This caused areas that did not allow good coolant flow, causing steam pockets, so the 261" block and gasket were vented to the head cooling jacket in 6 places.
The 235" gasket doesn't have the 'steam hole' provision so you must use the 261" gasket when modifying the '848 head.
