I bought a World Engines 540ci short block from my local speed shop a couple years ago. I wrongly assumed it was internally balanced and bought a 427 Fluidampr specifically for that (and 427 flexplate). after startup with lots! of vibration, I emailed the company, they weren't sure whether it was internal or external. damn thing shook like an out-of-balance wash machine at idle.
so on a guess, I installed a 454 Fluidampr and 454 flexplate instead, and it's fine. Their web-pages for this motor said internal balanced... I was lucky to have the 454 parts laying around.
The motor assembly quality and parts seem fine, no issues there, just their customer support wasn't very helpful, even when I provided them the serial number.
Other than that, I'm satisfied (i.e. I've gotten over it).
assembled with Edelbrock Performer RPM rect port heads, RPM Air Gap, Crane solid street roller cam.
I would buy another one.