If you look at the Sierra Nevada mountains in California, that's an example of a large batholith that's being pushed up as part of a collision with the North American continent. There are a lot of other things going on in that area and it's hard to get things like that happening in isolation. Note that Yellowstone is a special case as it is a hotspot burning through the crust (see the Snake River Plain for an example of what it did on the way to where it is now).

A large batholith like that would need to be emplaced from something like a previous continental margin. Only much later would something push it to the surface. The fun thing about that sort of behavior is that you can get nice linear features (see the Kern River Canyon in the southern Sierra Nevada mountains in California) caused by breaks in the batholith.

As far as the valley goes, is your intent to have it get narrower or wider as it heads to the sea? I will admit to being curious about its significance and whether it needs steep walls and a flat bottom or if a more traditional V shape is as useful?