I've installed two of Dave's adapter plate (both on 261s, 1959 &1960). No leaks/problems with either installation.

The thickness of the plate is correct, except as Dave notes in his blog site:
"*Note: When installed in a 1939-40 Chev, the fan will be ¼” forward from stock but will fit fine."

http://chev235guy.blogspot.com/search/label/Water%20Pump%20Relocation%20Adapter