Help identify please: power or manual gearbox?

Yep, that's a PS box. IMHO, it would not be wise to hook up a pump until the box has been cleaned out internally. Hydraulic pumps do not take kindly to debris going through them. If the interior of the pump is scored, like an AT it will growl or whine.

The PS boxes are lots faster going 3½ turns lock-to-lock where the manual boxes are 5 turns.

The Saginaw pump is a bit thinner than the Federal Mogul and will move a bit more fluid. The thinner size makes for a little more flexibility in terms of where it can be mounted.

The 73 and later uses the same Pittman arm as the F/M/J bodies. I've got a cop car box on a 75 k-frame in the Demon. I'd recommend the cop car box to anyone who wants or needs power steering.