What needs to be done to make the 911 speedo function with a 915/61 that had the diff flipped? The pickup needs to be on the other side, right? Where should it go and how?

I am guessing here...but it may give you a good place to start looking.

If its an electronic speedo its probably using a hall effect sender.
With these senders the magnet is in the speedo and could just be a tab on the ring pinion shaft or drive shaft at the back of the case...not sure.

Take a look at the sender and see what metal tab passes through the sender body to create the magnetic field...

Rich

You can machine/epoxy a pocket in the side cover for the stock pickup/sending unit.
I have pictures somewhere that I'll try to find for you.

Or you could mount magnets onto a cv joint and make a bracket to support the pickup nearby.

The magnets are carried in an aluminum plate that is pinned from rotating and held down by the bearing. Be sure to check or renew the epoxy that holds the magnets in place. This is how you move the sender pocket to the other side.