Forget the 250. The low deck Chevy 6s have a head bolt that goes into the area behind the water pump, causing a weak spot. The 292 has a 'high deck' and the head bolt doesn't go into the water passage.
Buy Leo Santucci's book. There isn't much it doesn't cover, including the fact that if you run your engine at less than 4000 RPMs most of the time, a carburetor larger than about 300 cfm will cost you torque, although it may increase horsepower.
