Factory hub caps on aftermarket wheels?

[Number21]

I have a '79 1 ton with the stock white painted 8 lug wheels and Chevy hub caps. Somebody replaced the back wheels with 16" instead of 16.5, they're just cheap white painted steel wheels. I have the full set of 4 caps, is it possible for me to attach them to the aftermarket wheels? If I hold the cap up to it, it almost matches the front wheels. I'm just not sure how I'd attach it? And, would I have to have the tire/wheel rebalanced?

I'm going to get a new set of wheels and tires eventually, but for now I want to spend all my cash on the engine.