Confusing topic, I thought 73-76 used 15x8" offset: -1mm backspace 4.5", Std OEM tyre 225/70R15.
So? if you go up to 18" dia - do the other diamentions stay the same esp if the width remains at 8" ?????????
Normally if you +size a wheel you aim to keep the new rim offset the same as the original.
So if its 15" x 8" 4" backspace today you would go to 18" x 8" 4" backspace.
If you went with a wider rim you would modify the backspace to have the rim so the increase in width is evenly distributed in and out.
This is not always possibly but it allows the suspension geometry etc to work as intended.
Go to this site to play around and it will show you visually what the changes are and explains how offset and backspace are related.
http://www.rimsntires.com/specspro.jspAs for 8" C3 rims I thought they were all 4" backspace