lost all oil pressure but not flow

You could also remove the distributor and intermediate pump drive shaft, install pump priming tool and see what you get for a result.
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do the above suggestion first, if its pumping, get another gauge and mount it in the engine compartment so you can watch it. We also had a new oil pump go bad on start up one time on a hemi, it happens . I`d run heavier oil too, these are dinosaurs, not sewing machines.