Weird knocking on 91 gmc

[wulffboy89]

I have a 1991 gmc c1500 with the 5.7 v8 with 449k on the original motor and transmission. I named his big boss. I recently had a bunch of parts replaced on my engine. Water pump, thermostat, upper and lower radiator hoses and most recently the intake manifold gasket. After they did the intake manifold gasket, I had them do a tune up. When I got my truck back, it has a very hard time starting and when I finally do get him started, he has a loud knocking noise with low oil pressure and a fluctuating idle. When I go to put him in gear he bogs down and dies most of the time but when I can actually drive it sounds like the knocking either goes away or gets camouflaged by the other sounds of the engine. When I come to a stop and the truck goes back to idling, voila the knocking comes back. I've done a fair amount of research and haven't come to a solid conclusion. I've heard rocker arms, lifters, camshaft, oil pump, steering pump and a bunch of other crap. I'm about to take the truck back to the shop and have them look at it again but I don't want to keep paying them 100 for troubleshooting fees. Any advice?