Would you also lower the height by a foot, decrease the width by at least that much, and lower the floor onto the frame to get rid of the basement storage? Generally, there are a lot more differences between "standard" Cs and those marketed as B+ than the shape of what is over the cab. Enough that you are not likely to find a B+ cap to bolt on as a replacement.
It would not be unreasonable to convert that overhead space to cabinets and/or entertainment equipment. That has been an option on many C models for at least the past 10 years.
