It is worth noting that MIL-STD-1553 can and has been retrofitted into multiple aircraft that were manufactured decades before it existed.
There's still part of me that can't quite believe that the SR-71 wouldn't be an exception. You don't just make a turbojet out of 1965 technology, without something being so wackadoodle batshit crazy as to make comparions to all other planes pointless. Another commenter mentioned something about a "hydraulic computer?"
I'd love to be wrong. It'd be hilarious if some madmen from the 1960s built a Mach 3 jet out of mostly-normal parts, and then that mostly-normal jet turned into American patriotic folklore. And yeah, I'd feel a bit of patriotic euphoria to see a Blackbird in the sky! I'm not above that.