I have a 100k 5.3 in my garage, bought from LQK last year and probably wont have time to actually use it for awhile. Inside looked pretty clean when I pulled the VC's off but that was about all I did.
Colin showed me the pics of the bearings, if most of the rod bearings were junk then I'd lean heavily towards oil starvation. Theres a rear galley plug (commonly called the barbell) that goes behind the rear cover, this sends oil to the filter and keeps the oil from just dumping into the rear cover cavity and back into the pan. If this is missing and/or damaged you will have shitty oil pressure. There is also a plug in the front galley but you have to hammer it in and usually not an issue. LS' are also known for spinning cam bearings, not super common but still can happen. Worn rod bearings typically do not cause a huge drop in oil pressure. I had a motor with two spun rod bearings that had 35psi at idle and 60 when driving.
