I am tired of horsing the wheels and tires around on my '72 stock 4 hole hubs. I plan to stick with a 4 hole wheel so wonder if its an easy swap to upgrade to a '73 or newer hub with a centering ring?
I may be adding flares and wider tires and wheels down the road but don't plan on any racing so I figure the 4 hole wheels will be fine.

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Early vs. late discs are not compatible, if you want to go with hubcentric discs up front, you'll need 73+ struts along with matching ball-joints and calipers. They're easy to find relatively cheap but it's alot of hassle and money for shipping.

You should consider making your own hole centering tool to install the wheel, basically a long bolt with the head cut off so you can slip the wheel on it to help installation. Here is some pictures of some mercedes tools.