Proper backspacing for 20”x12” rear rims

[Pure-Juice]

Hello. I have a 1969 C10 with the narrower rear end and looking to see what backspacing I would need to order a set of 12” wide wheels. The truck will be a fleet side. Currently there is no bed on it so I am unable to take measurements. I will be mini-tubbing the new bed to accommodate the wider rims and tires.

I have done a ton of research on here before posting but I’ve seen answers all over place. From what I can tell, the consensus is 7.5”. I’ve read that is heavy on the conservative side. I wanted to see if anyone is running a 7” or even a 6.75” with success.

Thanks in advance for everyone’s help.