Looking at the pics again, the top deck also splays out further towards the rear. This is needed to allow the motor to be 1mm closer in and have clearance with the top deck:
On the WE, the motor actually butts up against the upper deck:
So to use the new motor mount on the WE, you would need to also use the top deck. And to use the new upper deck, it looks like you might need the new rear lower bulkheads and layshaft support so all the holes line up...