You may want to check the car base plate gasket and the brake boost line that goes to the intake, it sounds like you got solid vacuum and everything but it is worth checking out.
Actually it's funny you should mention the carb base plate gasket 'cos I have a couple of questions about that:
Which way up does the carb spacer go on a 258? Does the recessed side face up? I've had it both ways but right now that's how I have mine.
The other question I have is: Is there a gasket between the carb spacer and carb as well one between the spacer and Manifold?
The carb kit I got had two of the same carb base gaskets (same as the one that I scraped off the manifold) and according to the autozone site two carb base gaskets are required but the gasket isn't really the right shape to fit the recessed side of the carb spacer. Iv'e noticed some slight leaking around the gasket I have between the carb and the spacer so I'm thinking it's either not supposed to be there or the the carb to manifold nuts are not tight enough.
Cheers!
tony and nicki