Center bore Dia. for Custom wheels???

[Gregsmx]

I am researching custom wheels on my Blazer and one item I'm unsure of is the center bore. The axles are stock, a Dana 44 front axle and GM Corporate 12 bolt rear end. Does anyone know the correct center bore that is required on the wheels?

I know this is a critical especially on the rears because the wheel opening fits snuggly on the axle hub.

After determining this, the wheel application is as follows:
20X8.5" 6X5.5 bolt spacing 5.125"backspacing.

The front axle is wider than the rear axle, and I'd like the tire footprint to be as square as possible. Does anyone know how much I need to 'spread' the rear wheels, and how to do it effectively?
Thanks, Greg.