Hey Phil,
I just did a side-by-side comparison of the Shapeoko mm TC.pp and the G-Code Arc mm.pp Post Processors. With the exception of the “History” section, they are almost identical. The Shapeoko PP has the ARC segments already in it and looks like it should be named Shapeoko mm ACR TC.pp.
Please check carefully… I may have missed something. 
I’m thinking if you match the two by simply inserting the
+---------------------------------------------------
+ Commands output for tool change
+---------------------------------------------------
begin TOOLCHANGE
“M6 T[T]”
beneath the Commands output at the start of the file segment you should be able to insert the tool change coding according to my recent post.
Please test carefully. Just my two cents worth. 
ps.
I’m 76 trying to get to 77.