1970 C20 8 Lug Wheel to Dually Style

[docrock]

I have a 70 C20 chassis for my next project. The current width, on the front, of the wheel mounting surface to wheel mounting surface is 65 inches. When I bolt 16x6.5 8 lug wheels to this I end up adding 7 inches to the width. Which is 72 inches from wheel edge to wheel edge. The tires will have a bit of bulge and another inch. I want to narrow this by 3 to 4 inches. My thought was to use a dually style wheel. Do you have to use a hub adapter like what come from the factory or can I use a spacer? And what size spacer? Or maybe there is different size hub extensions, or?
Thanks for any help