I have a chance to buy a M-21 a guy pulled from his GTO. I want to put this into my '66 Chevelle. What issues will I have with a muncie from the Pontiac GTO?

Thank you!

None, none, none and none.
The 63-EARLY 65 Muncies in Chevys had the speedo fitting on the driver side of the tail housing. In 65, The speedo fitting (in Chevy products) was moved over to the passenger side of the tail housing. Some Pontiacs continued to have the speedo fitting on the driver side up until about 1970. Thus, that Muncie from the GTO may have a driver side speedo fitting and your speedo cable will reach it just fine.
Otherwise, a Muncie is a Muncie, is a Muncie. :thumbsup:

Thank you Tom for the quick replay!