Hi James, Not sure where you are located (not in your profile) but I am a big fan of the awning windows since they can be used even when it is raining. The website you listed clearly carries CR Laurence windows but I did not see the FW904RS shown.
They are $425 plus shipping or will call at CRL, I can order for you if you like or you can ask how much the website you listed wants for them.
Reference point for window install on the Econoline is the lower line. The window should be parallel to this lower line. Drilling a couple of reference holes in the middle of where the window is going is an excellent suggestion. The inner wall is often not where you think it is looking from the outside! Lots off masking tape and very sharp fine tooth jigsaw blades on a very good jigsaw makes cutting easier. There is not much leeway for errors in cutting. The window flanges are not very large. Lots of patience, measuring and drawing with a marker to make sure it is all correct before actual cutting.