If you had the 71A box fitted before you've converted it to the 71C box, you'll need to have a new tailshaft made regardless,
as the 71a box has an output flange (as opposed to a slip yoke on the 71B/C box's) and a sliding spline centre section.
Personally, unless you installing a 250hp+ motor, I would have kept the 71A flange drive box...