I am 98% sure it will be the correct length. I have the new yoke (sp?) though for my driveshaft. I also have to get solid UJoints. I know the Muncie and TH350 had the same length DS and From what I understand a Muncie and a ST10 are the same length.
As long as the output splines are the same for both trannys. I seem to remember the ST10 and Muncie using a TH400 yoke...