I've studied all available Black Album footage since I was a kid like the Zapruder film or something, because that's what got me into recording. My favorite guitar tone and overall production ever.
They had 4 4x12 cabs set up side by side (with a little distance in between them). Randy put a 57 and 421 on each cab. Back in the room they had a a stereo pair of what looks like 414s in front of the setup, then further back and way up high they had some sort of condenser like an 87 or something. Don't forget the "tent of doom," which was a tent of sorts, built from foam and moving blankets, to enclose the cabinets to keep the sound tighter. The 87 (or whatever it was) was up high aimed at an opening in the tent of doom. Also, don't forget that while the Mark II C+ was the main part of the sound, they also used an ADA preamp and who knows what else (Bob and Randy are big on blending amps when recording: see
THIS LINK).
Good luck and let us hear how close you get! I've spent my whole life trying to nail it.
Matt