Kilroy,
As I don't have a TF truck, I can't directly answer your question from personal experience.
I do have access to a TF (55.2-59) FAM (Factory Assembly Manual) in PDF format though. I took a screenshot of the diagram for a 3100 frame (attached below) and it doesn't show an actual "transmission" cross member. Just the "engine" rear cross member that supports the bell housing/transmission.
From my knowledge gained from fellow Stovebolter's, your existing 3 speed (SM318) wouldn't have a pad for bolting up to a rear cross member, unless you have a non-original 3 speed. The transmission is actually fully supported by the bell housing and rear engine cross member. Fellow Bolters, please chime and correct me if I'm wrong!
That being said, installing a Muncie M21 directly off the existing bell housing should be a fairly direct bolt up from what I was able to read in this old thread in the HAMB about
comparing 3 speed and 4 speed transmission dimensions and installation.
As to if the M21 will need a supporting cross member because it has an aluminum case and may be subject to more stress if unsupported at the tail housing, that is another question entirely.
Am sure that fellow Bolters that have already done something similar will chime in with their knowledge and experience soon.
Let us know how your swap progresses, with pics if you can please and thanks!
Dan
EDIT: Mike B beat me to the punch! Thanks Mike
