Rob, what are these brackets from? I dont know, all I know is that the 340 mount I have doesnt look like the one in reference pic. The one in reference pic has quite the curve to it.
The ones in the top pic are B/E body and DO NOT work...
The one you have in that bottom pic is the correct part for an A body. If you want to look it up. Section 9-64-0 in 1969 parts manual, text page 9-12, or about page 325 in the PDF online.