The pump and brackets look like '73 and up parts like these. Part 16 was confusing, never seen one with the square drive hole for tensioning the belt before.
Here is what I could find on the water pump pulley.
https://www.chicagomusclecarparts.co...ed-70-74-chevy
Looks like its possible the power steering pump and water pump pulley came from a 250 6 cyl from a car.
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'51 South Bend Model "9A"
'56 Chevy 3600 NAPCO
'74 Chevy Cheyenne Super K20-restored on 37's
'72 GMC Sierra Custom K2500- in progress
'08 GMC Envoy Denali
'12 GMC Sierra 2500HD Denali
'17 GMC Yukon XL
VCCA member #58596
http://www.nohrco.com