For a 250, I don't know, because I don't know the port configuration in the cylinder head.
For a Chevrolet 261 using downdraft carburetors:
Maximum power plus best economy is going to be acquired by using 3 SMALL CARBURETORS on a three carb plenum type intake with equalizing tubes.
For the 261, with some internal modifications, three Carter W-1 tag number 420s is going to difficult to beat.
The carbs should be run with solid linkage (3 chokes, 3 idle circuits).
For a highly modified 261 for racing only???, you can make three Carter W-1 tag numbers 483s, 574s (these 2 may be mixed or matched) or 684s (no mix). I would NOT suggest these numbers for street.
Be aware that the stock metering rods may need replacing You will not know until you hook up a wide-band to the engine.
Be aware you MAY have clearance issues.
You might also consider the Corvette 3 carb set-up which was factory engineered for the 235.
Be aware you may have clearance issues with this set-up as well.
For MODEST gains in both HP and economy (DEPENDING ON YOUR TUNING SKILL) 2 of the 483s or 574s will be easier than 3, again with a 2 carb plenum style manifold and equalization tubes. Again, the metering rods may need to be replaced.
Jon.
