If you have not put the pan back on, you need to pull that high volume pump out and return it for a standard volume pump. As dirty as your engine is, that HV pump will put all your oil on top where it won't be able to drain back. Chevy engines don't need a HV pump unless they need to feed an external power adder that requires pressure lube.
Pressure should be created by proper clearances in the engine, not increasing the oil flow. HV pumps put unnecessary stress on the cam and timing set. Hot Rod magazine had a little blurb several years ago where they dropped the oil pressure 3-4 lbs and picked up about 7hp.
The proper pump you need is a Melling 77A, which is the Z28 pump. It is standard volume, but the pressure relief spring is stronger and allows for more pressure to be built before opening the bypass. A HV pump runs with the bypass open most of the time, heating up and aerating the oil, reducing its ability to protect engine bearing surfaces. It also greatly increases windage which robs power as well.