Swapping in a 66RFE (in a 2010-2011 truck) would require new PCM software to enable the correct shift pattern. And that creates a concern for keeping the software and hardware matched. Suppose you installed a 66RFE (with new PCM software). A year later, you go to your dealer for an oil change, and he "helpfully" updates your PCM to the latest level of software (for a 545RFE truck!!). So there are some issues with doing this.... but I'll see if any of my colleagues think it might be a possibility.
Thanks for the info and looking into it.