If you're bringing the WPB poncho, (which i'm assuming is actually Frogg Toggs?), you don't really need a bivy if you bring a small piece of polycryo for beneath you. Should save you a few ounces.
Especially since your tarp is so large, a true bivy is less necessary, and if you do happen to run into some weather that is really blowing it in, the WPB poncho will protect you just fine.
Add a few tyvek tape loops, like BearPaw Wilderness designs sells, to your poncho and you can easily secure it with some cord.
Another tip for the Frogg Toggs Poncho, is to bring some tyvek tape. Sticks really well to it, is pretty weather and water resistant, strong/durable, and lighter than duct tape. Chances are, you will get holes in the poncho.