Not to worry in the least about the fit! As I understand it, the earliest mantuas did not have the plates sewn down - I expect they would have fit the same way. The front pleats are as important to the shaping as everything else, and make a huge difference in the finished appearance of the mantua. That said, I would leave them until after you stabilize the neckline, as your front pleats can have a great range of fit to them, to be as tailored or loose as you prefer.
You should be well pleased!