99% are form the same Chinese factory. No matter which vendor. Proper fitting depends on what pieces. Some will fit great. Some may need tweaked. Light tweak to heavy tweak. Your truck can be the blame for some bad fits. The parts can have handling issues, shelf, ships hold or container stacking racking/bending. (large pieces especially). They can have poor Chinese quality control or worn tooling. All in all they are way better than you can make. (Unless a smallish simple piece or you have experience and tools and metal). Not enough demand for USA made. I have purchased patch panels for my 60 GMC (same truck as yours) and other trucks, at Classic Parts of America in MO. I have also used donor parts for patches. List all the panels you need and that will help others advise. There is at least one company in US that makes patch panels for some trucks. Never checked on them.

Right is right, even if everyone is against it, and wrong is wrong, even if everyone is for it. - William Penn