Good catch. You're correct the PS pump is stationary. I failed to mention the belt for the PS pump also drives the water pump and alternator. The alternator will be the tensioner for that belt.

Here's the same setup on my 57 Plymouth.

[Linked Image]HP4 by M Patterson, on Flickr

1957 Plymouth (Hemi, Dual Quads, A833 4 Speed 9 1/4 w 4.10) Sold
1937 Dodge Pickup (Hemi, 6X2 intake, 46RH, Dana 60 w 4.56) Sold
1968 Plymouth Valiant 2dr sedan (354 HEMI, 46RH w/4.30 gears)