I dropped a 383 with a mild cam into my 1986 K5. I am using a Holley Street Avenger 670 CFM with MSD ignition. I have been using 93 octane. The engine is a crate motor that was first cranked about 14 months ago. The engine seems to run perfectly except when it is under load. When I'm out on the road and accelerate, she rattles like she's drinking bad gas. I suspect the torque converter may be to blame. The truck doesn't seem to have the power I think it should have. Could the TC be causing the engine to labor and rattle? I put a switch in to manually control the TCC lockup. It was worse when the clutch was in lockup. I hardly ever engage it now. I don't have the original specs on the motor so I set the timing with vacuum. I have tried adjusting the timing back, but there is no improvement and the engine doesn't run as smooth. I appreciate any thoughts from the forum.