6-car Pendolinos would be ideal for the route, in my opinion.
With one first class carriage (smaller kitchen to allow more seating the driving vehicle, as, let's face it, the current catering offering is a trolley so anything would be an upgrade!) you would have the same standard class capacity as a 9-car Pendolino (before the recent change that has seen coach G declassified).
I know that it is voiced frequently on this forum, but Manchester to Scotland would sit far better with the West Coast franchise than with TPE if the stock was available for it. It seems a natural bedfellow to the Birmingham - Scotland service. I'm quite happy with 185s on the "proper" Transpennine network and the North West express services, but these trains, and the similar 350s, don't seem to be properly suited to the Manchester to Scotland service compared to the Voyagers and Pendolinos that also ply most of the same route.