more pan capacity
Yup. With such big displacement, and the original case volume, the pressure spikes just from piston motion is high. More volume, anywhere it can be hidden, is better, but it's got to be connected directly to the pan/case space (not the valve covers).
Re: pump inlet pressure vs. case vacuum.
By definition, a vacuum system is not vented to ambient. The entire hydraulic system (suction, bypass relief valve, and bearing/tappet/rocker leakage) is vented to the same pressure: the case. The differential between suction and pressure is the same regardless of the starting value.
However: even though the pump delivers exactly the same pressure and volume (but not on a gauge balanced to atmospheric pressure!), as Rex says, the small change in pressure is far more dangerous on the suction side.
Doesn't this suggest that the entire suction side (pick-up head, tube, passage in the pump body, low-press port) should be larger X-section as compensation?