Sure would be nice if the instructions:
https://documents.holley.com/199r11655.pdf
Listed the part number for the spacers for the different transmissions. It was by dumb luck that I discovered the 11/16 spacer for the 4L60 is PN 71223012HKR. I figured they might be included with the crossmember. But, for $30+ I'll be machining my own spacer.