I would assume they could be hard to find. Need to find a salvage ’61 to ’63 that had A/C. It was not a very popular option at $378, more than 10% of the total car’s cost. I would think the Buick’s shared the same or similar brackets when the 215 cid was ordered vs the 231 V-6, but the Pontiac offerings sold very few 215 cid”s. You might be able to get an old Hollander salvage yard cross reference book, but I don’t know for sure that they covered brackets?