The top two pics are the incorrect orientation for the biscuit mount holes. The bottom one is CORRECT!
These are 340 A body brackets that fit a 67-72 biscuit mount PERIOD, no doubt... Drivers side on left...
View attachment 1715478101
Your 273 driver side needs a tab to pick up the upper mount boss on the block.
3 Pages of confusion because there is a crap load of poor information on the net.