Engine shakes under acceleration

Had a friend have a similar issue with a 360. They were using the wrong flywheel. The shaking was there the first time they fired up the engine after installation. It became noticeable after about 2500 RPMs. Is the engine newly installed or has this been working without issue until now? The issue is, he was using a 318 flywheel. A 318 is internally balanced. A 360 needs and offset balanced flywheel