Making WWPA 105 Shiplap siding

How many lineal feet do you intend to run? A shaper would be a better tool if you are going to be making large quantities of siding. A router could be used with a panel rising bit, then cutting the rabbet on the table saw with a dado will be faster than using a router. Here is the type of bit you'd need. You will want to slow your router way down when using large bits like that.

I use this bit to raise panels for interior/exterior doors. The radius of the cove is 1-1/2", slightly greater than the 1-1/8" that the WWPA standards call for.

Keep in mind that this is a 3-1/2" diameter bit. Be sure your router table can accommodate it. Like Bondo said, you need to slow your router all the way down for a bit that big. You might need to make your cuts in multiple passes, depending on the horsepower of your router. A feather board mounted to the fence will help keep the wood flat against the table top.

What about vertical bit, like this
.
https://www.amazon.com/Whiteside-Router-Bits-5801-Vertical/dp/B0012JCUII/ref=sr_1_7?s=hi&ie=UTF8&qid=1492908928&sr=1-7&keywords=vertical+cove+bit