Your '66 block won't be correct for a '66 vehicle.
Although it's a '66 casting, it appears the block in question came from a '68.

https://board.moparts.org/ubbthreads/ubbthreads.php/topics/2045293/426-hemi-block-id.html#Post2045293

A block from a '68 into a '66 is still more correct, at least in appearance, than one of the aftermarket blocks from the last 20 years.