kylecanthurdle

hey everyone. im new here but not new to cars/trucks and forums. ive searched and searched for the longest time tryin to fix this van but i cant seem to figure it out. ive got a 2005 chevy express 1500 with the 4.3L(vin X). its got almost 230,000 miles on it. i am a mechanic working on this truck and have just completed an engine swap since it cooked cylinder number 6. after finishing the engine swap it ran like garbage. was getting a MAF code and a P0300 random/multiple misfire. hooked it up to my snapon comp and it was misfiring in 2, 4, and 6 equivalently. changed the MAF, still did the same problem. then changed the cats since they were so clogged. this seemed to fix the problem for a little bit. no codes, ran like new. the van came back today and ive been diagnosing why i got another P0300 code. snapon scanner says cylinder 1 and 4 misfiring when revved above 1000 rpms. cyl number 4 has about twice as much as cylinder number 1. keep in mind its got new plugs, wires, dist cap, rotor, ignition control module, ignition coil, cam sensor. checked the distributor since its the plastic POS. i just cant figure this thing out. anybody know what this could be?