A scroll chuck probably has as much internal friction as it does actual holding power, so maybe you get half of what you think you're getting when really reefing on the wrench.
For grabbing stuff like solid shafts, I like to use a stacked pair of aluminum vee blocks (vees facing each other) in the hydraulic press. This means working with the part axis horizontal in aspect, but that doesn't usually matter to a nut and wrench. The aluminum is soft enough to deform to fit the shaft without raising dents. 10 or 20 tons of pressure provides pretty good grip. No fancy Vee's required, they get messed up anyways. Cut the vees with a bandsaw if you have such available. The 'natural' angle of Vee blocks that have been used extensively for shaft support in a press seems to squash out to about 115 degrees (started life at 90°).
If you really need a vertical axis, maybe rig up something with a couple of support pads opposing a portapower cylinder ram.
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