I've read elsewhere that using an adapter plate to use a square bore carb on a stock spread bore manifold costs power. I've searched this site and found only a couple of passing references. So, better of to match them up properly, right?

 

Yes, its better to match carb to intake. Many intakes have dual-pattern setups, that can work with a spreadbore and squarebore carb, like the Edelbrock RPM-Qjet intake, and they are fine. The spacers can cause some weird running issues though. I'd avoid using the adapters if possible.