I'm rebuilding a 68 Muncie for a customer and this is typical of what I find inside. The 63-69 Muncies did not have a drain plug (except M22s). But ALLLLLLLLLLLLL Muncie cases have the boss for a drain plug. The 63-69 Muncies have a magnetic donut glued to the bottom of the case. There is NO WAY to drain the tranny, nor is there any way to remove the accumulation of metal particles from the donut magnet (without removing the tranny).
This is typical of what I find inside a hi-mileage/well used Muncie. You can see the outline of the donut which is covered with sludge/tiny metal particles.
Same case with all the guts removed.
The removed and cleaned donut.
As I mentioned, ALLLLLLLLLLL Muncie cases have a boss for a drain plug.
I start out with a pilot hole to assure it will be centered and straight in the boss.
Then drill a larger hole (11/16 drill bit).
The hole is tapped with a 1/2in pipe tap.
With a magnetic drain plug installed, the oil can be drained and changed, BUT, it also permits removal of any metal particles which can continue to circulate in the tranny and cause continued wear to gear teeth and bearings.