Well that seems to rule out the measuring device as culprit. Some other ideas... filter differences (some filters have internal bypasses)...pickup design (I've read that the larger circular stock type pickups are more prone to generating vortexes and sucking air if the level drops too much) though you're using a 6qt pan and std vol pump so that doesn't seem too likely.
If you do choose to replace the pump, here's what I used on the SBC oval track engine I refered to and it seemed to work out great;
Melling M55A - this is their equivalent to the Z28 pump. $30 thru Jegs. Melling tech says its good for about 10% more volume and pressure than the standard pump. On the dyno it produced very consistent pressure and the bypass appears to open at about 80 to 85 psi (85 when oil was 180deg, 80 when oil was 220).
I was using Valvoline VR1 20W50 with a blocked bypass, bearing clearances were .0025, NAPA Gold 1061 filters (used for 3 seasons of oval track with no problems - changed after each race of course), Moroso 6qt road race pan #21900 and pickup #24170.
The only change I made to the pump aside from disassembly/inspect/clean was to set the clearance between gears and cover to .oo3 by sanding base cover and gears on a flat plate with 400/1200 grit.
btw - what heads, cam and intake did you use on the engine? - you got great results!
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