I wrote a tech artice on various carbs I tried out on my Chevy 261 engine for Inliners Newsletter.The engine was in a good state of tune,3/4 cam,Fenton headers,more compression,proper valve job,minimal cylinder leakdown.A 250 Chevy breathes better than the 261,but the comparisons should hold true for both engines.
All carbs were on a Clifford water heated intake,Carter(expensive) 400 cfm,Holley 390 4bbl,Holley 350 and 500 cfm 2 bbls,500 cfm Edelbrock.The carbs were jetted if necessary for best cruise vacuum and wide open throttle performance.Part throttle response was judged by by "feeling",wide open throttle performance was timed on a back road, between 2 power poles using 2000-4500 rpm in second gear.
In the end,they all performed pretty much the same,but the Holley 390 4bbl offered better midrange respnse due to it's easily adjustable vacuum secondaries.It was the only 4 bbl that you could quickly floor the accelerator pedal at 1000 rpm in first gear,and it would pull to readline with seamless power.No real difference in fuel mileage between carbs for my tests.The Holley 350 and 500 2bbls(about 270 and 370 cfm respectively if you rate them as 4bbls),ran well,but the 500 2bbl may be slightly large for slow speed work depending on your engine tune.
A few guys on this site run Holley 390 and 450 4bbls I rebuilt for them on 292 Chevy engines,they seemed to be satisified,like I didn't get any death threats.
60's Ford V-8's and 266 and 304 IHC truck engines use a Holly 2 bbl that's more or less similar to to a new Holley 350 2bbl.You can pick them up on the cheap from Ebay.A good deal if you're capable of rebuilding a carb.The stock jets in these carbs work out ok on a Chevy 6.
I also tried out a large Rochester 2 bbl 2GC,I was not satisifed with the performance,and time ran out before I jetted it properly.
I hear some guys say Holleys always leak fuel,they do if you're ham fisted,it's nice they don't have fidgety little springs and metering rods that always fall into crevices when doing jetting changes with carb on engine.
