There is a good article in the latest "Home Shop Machinist", available now
in good bookstores, on a device to automatically disengage the half nuts
at the end of threading. It's very clever, but I don't know if it will
work on a South Bend. The exact reference is:

"An Automatic Carriage Stop for Thread Cutting" by James McKee M/J 05 HSM

If you want something that will prevent power-crashing the carriage into
the chuck when powerfeeding, then you need a limit switch setup, perhaps
backed up with a solid beefy carriage stop. I don't know where plans for
a limit switch setup are.

GWE