2001 GMC Jimmy multiple misfire code

Ok here's the back story, my aunt got this truck from a friend who took it to the dealer to have it checked before giving it to her, and the tech told them it was ok but shouldn't take it on long trips. So I take a look at it, the check engine light was on, had code p0300, and ran smooth at idle, but felt misfire when under load. Also noticed the oil pressure gauge read 25 psi at idle when hot and 60 when under load (which, in my experience, is about right with gm gauges) Replaced plugs, wires, cap and rotor, cleared codes. Truck ran great, no misfire. She drove it for a few days and the check engine light came on, I scanned it and it came up with p0300 again, but was running smooth at idle and under load, I didn't have time to look at it further so she drove it from Richmond va to baltimore md and back (about 300 miles) and when she got back it had a really bad rod knock. So I replaced the engine with one from a junkyard {the vehicle was wrecked and was told the engine ran) now the new/used engine runs great but the check engine light still comes on with p0300 code, I have a matco scanner that reads real time data and it shows misfire on cyl # 1 & 4, more times on 1 than 4, I replaced the ignition module in the distributor on a hunch, but didn't change anything, I'm going to check the CKP and do a fuel leak down test, since I heard they could be probable causes. My question is, how can I check for a weak injector or do you think the rod bearing in this engine is starting to go bad as well, if so, how can I diagnose it, without pulling the oil pan