I currently have a set of Rivieras (with VW offset) on my '75 914. They have 185/60's and they don't rub on the rear wheel well lips (but then again, the lips have been rolled under).
The 914 they came from
did rub a little on the rear, as evidenced by small gouges on the outer diameter of the tires. Be aware that some 914's have wider 'gaps' between the suspension components and the fenders on one side or the other - from the factory. Each car has the possibility of being 'slightly different'.
Also, individual manufacturers' tire widths vary, even if they're listed as the same exact size, so it's good to do a little research before purchasing.
As for the center caps, IIRC I purchased them from a seller on eBay (ISP West?), but I've seen them available at a couple of local tire stores, as well as a few local VW Bug/Bus aftermarket stores. I purchased the Porsche 'stick-on' inserts on eBay as well (Adante)?