This is the adapter part you will need if anyone is curious.
Motive Products 1115 Large... https://www.amazon.com/dp/B00YS8JOS4?ref=ppx_pop_mob_ap_share
Old one was leaking* no way to test. Now has become wall Art/ paper weight. I could only give data on new one after install. It truly is a work of Art. Thanks again Radiator Technologies!
I ended up getting a new one made from Radiator Technologies in FL. It's 1/3 bigger and the filler neck is Now the same size of a Radiator filler neck to make things easier when filling and finding another cap if needed.
Just do one size* then you can rotate them. I did staggered fit....it was a PIA. It looked cool but wasn't that functional. But hey...you do what you want. I just ended up sources two sets of c5 wagon wheels and used the 17s and sold the 18s off.