Over on LS1tech.com there's a post that says it the barbell is left out that the pump just dumps oil in the pan and you have no oil pressure.
https://ls1tech.com/forums/generatio...hematic-2.html
So like 71CADC10 says, you'd never see any oil pressure if this were the case.
There's also a galley plug under the front cover by the oil pump that has to be there. Did you have the cam bearings replaced? I've heard of cam bearings 'walking' out of the bore and dropping oil pressure but not actually seen one