help with new windows

I've just purchased two new construction windows with the intention of pulling off my vinyl siding, putting up a moister barrier, installing the windows and putting the siding back on. The problem is that the old wooden windows has around a 1-1/4 trim around the window while the new one's don't. And if I just move the j - channel over to butt up along the new window the siding will end up being 5'' to short.
Could I fill the gap with some pvc trim board and just apply some caulking along the edges or is that a bad idea. I don't believe I can return the window's as they are a custom size. And I don't recall seeing any aluminum clad windows with any exterior trim. Any advice would be appreciated thanks. Michael

 

If you happened to purchase Jeld-wen vinyl you can order a flush fin adapter to attach to the outside of the window. That could possibly reduce or eliminate the gap. I can't remember exactly how wide it is.