Many people running an 8" wheel (9" wide in reality based on how wheels are measured) run anywhere from a 4.25-5" BS in the front, and 5.5 for a 10" on the rear. That said, your rear may be narrower than some of those running a 5.5BS for the rear, as many of the threads are 71-72 C10s which have a slightly wider rear end.
What I'd do is borrow or buy a wheel mock-up tool and check it out. I borrowed mine from Matt's Classic Bowties since I was ordering my wheels through them, but I imagine there are other vendors out there that loan them out too.
I then had a mobile tire company bring out 3 different sized tires so I could mock it all up. Once I knew what I needed, I ordered the wheels and tires and was pretty confident they'd all fit well.
The wheels you selected look nice. I'm having a hard time getting past their website which looks about 15 years out of date
.