99 Astro AWD Lower end knock

I have just purchased a 1999 Astro AWD with 165,000 4.3 VIN W. He took the van into a repair shop and had then change the lower intake gasket (dex-cool caused it to start leaking) Once he got the van back he drove it for 2 days with lower than normal but tolerable oil pressure. Instead of the normal 40-idle 60-driving it was 20idle 40 driving. On the third day he was getting on the the highway and noticed it lowering towards zero so he pulled over and as soon as he stopped it started the engine started to knock. The engine only ran for about 4 seconds knocking. He then had it towed home and a few days later started it and it came up to full oil pressure and was not knocking. He then drove it from his house to the shop that did the lower intake gasket which was 1.5 miles from his house. It drove there fine but as soon as he pulled into the lot it lost pressure and started to knock and he turned it off immediately. He then accused the shop of getting debris into the pan and made them pull the oil pan. They showed him the oil pickup tube that was not clogged and said it wasn't their fault. I plan on purchasing a reground crank and matching bearing set and a new oil pump. One of the rod bearings did spin but it doesn't look bad at all, I have seen much worse bearings. But if I need to replace a rod is it possible to pull the heads and pull the piston and rod out of an Astro van with the engine still in the van? Do you have any idea what could have happened to the oil pressure after we got the van back from the shop? Thank you in advance!