A Body Disc Brake Swap 101

Im no smart man here, but I just removed all this from a 73 car, and I have new 73+ ball joints will they thread into the 71 UCA? I know the joint is bigger on the 73+ I'm just curious of they will screw into the 71 uppers or should I have removed the 73 uppers?
I read through and it sounds like I have to use the 73 UCA but I guess I'm just verifying I understood correctly.