68-70 b-body wheel well design should be similar, but i'm not an expert, there may be slight differences between makes and models. I would like to say 'yes, you're good' but even different brands of brake kits / rotors can add or remove track width. I'm running Baer rear discs (Cobra) and a dr. diff stage IV kit on the front.
All I can say is this wheel backspacing works on my b-body and an a-body application as well.
Here is a chart that may help you! May be beneficial to grab a few measurements before you order like others have stated.
I would be sure to give US Mag
a call to verify if that specific wheel will allow proper brake caliper clearance with the kit you plan on purchasing in the future.