Hi Doug,
I installed a complete seal set in my power steering gearbox last year. I surmize that your leak is around the bottom of the pitman arm shaft. If my memory is correct there are a couple of o rings to install. I think there is also a oil seal that is pressed into place from the outside. If you have the same parts manual I have the o rings, oval seal and oil seal are pictured in the top center of the page. I think the could all be replaced without removing the entire box. Would want an expert here to verify this before proceeding. Drain pwr steering fluid. Remove the piman arm and unbolt the the bottom piece which holds the seals. Install new seals and reassemble.
I'll be interested to see if others think this can be accomplished with the box still on the car.
Jeff C.
1970 280SL 4-speed