Not sure on a W-Engine, but we used to drill and tap our Small Blocks for the Mounts to fit in our '55 to '57 Chevys years ago. Best I can remember we would drill deep enough for about a 3/4 Inch Long 3/8ths Bolt to fit into it.
To be sure, if no one jumps in, I would check with the boys over at 348-409.com
-Parting Out over 75 '58 to '73 Full Size Chevy Cars-
-National Impala Association
-Vintage Chevrolet Club of America