I recon 280 hp is possible using street fuel if you use the right cam, the right grade petrol, and the right modified 500 cfm 2-bbl.
Ultraflow made some wild claims on the Sprint intake, over 300 hp with a 2-bbl Holley. Oval trackers are doing 305 hp with ease from a methonal burning 250 with Sprint manifold.
It is possible to go from 354 cfm (500 cfm rated at 3.0" Hg), to 460 cfm (650cfm @ 3.0"Hg). That's about a 65 hp boost, if all else is optimised.
The normal limits for carb hp are to divide the rated cfm by 2 to get power. (If you want to be pedantic, the rated cfm at 1.5" Hg, divided by 1.6 gives the maximum power.)
Holley made a 650 cfm 2-bbl carb in the 70's, it had the 62.5 thou over venturis and throttle bores, and a Dominator style air horn. Result was enough of an increase to make a 400 cube big block engine put out a lot more power. It was removed from the Holley range when the US oval trackers started having to use production stock 2GC Rochesters.
These days, BG fuel systems have a 500 cfm carb with the same 650 throttle bores and ground out venturis with a special plastic air horn. It's likely to make a lot kore power than a stock 500 cfm 2-bbl.
The best option is to have an Ultraflow Sprint intake, and pop in a custom grind cam. It is possible to get a 289 doing 354 hp with a 500 cfm 2-bbl, but its Brake Mean Effective Pressure is not very good, and it has to pull huge revs (6800 rpm) to to so. That means if you can make the engine rev, and custom make a cam grind to do so, you can get well over the normal limits with the 500 cfm carb.
On a 250, you can't go over 6500 rpm with a 12 counter weight crank from a pre AU Falcon. The stoker used from 71 to 93 to is limited to about 5400 rpm.
