Which steering box?

[turdmagnet]

I want to replace the steering box and I have narrowed it down to a couple choices at American Classic Truck Parts. Does anybody know what the right specs are that I need for a 74' K5? It has power steering. Does anybody know of a better place to get a steering box from?

SB7P-1 1969-76 Steering box, power, 4WD, 3/4 inch x 30 spline shaft, rebuilt


SB7P-3 1969-76 Steering box, power, 4WD, 13/16 inch x 36 spline shaft, rebuilt