Pretty easy. If you run two 1-bbl carbs, its jetted just as you would jet one 2-bbl 350 cfm Holley carb would be on any modified 250.
Depending on how big you can get the 1-bbls depends on how much performance you'd get. 250 hp requires 6000 rpm with that combo, so realistically, 200 hp at 4800 rpm would suit the 250 engine nicely.
I'd personally try a 1-bbl carb with enough venturi and throttle size to emulate one 2-bbl 500 cfm Holley 2300 (4412) carb, with 1.375" venturi, 1.6875" throttle, and then you could aim for as much power as you could with that venturi size. About 350 hp (flywheel, net)is the limit with one 500 cfm Holley and the very best intake manifold and cam to suit.
If you can't find carbs that big, find the biggest 1-bbls you can get, and copy the 350 cfm Holley 2300 (7448) ventur of 1.1875", and 1.375" throttle if I recall properly. As I've said, the best power you could expect would be 250 hp flywheel net.
You'd be looking at two F150 truck carbs at the very least.
Cam selection should be ground based on an Engine Analyser program or some cam builders custom input. I've seen some amazing numbers from two venturi carbs on 289's using cams designed to run the two venturis at very high engine speeds. Typically, they have poor low end response and require the engine to rev quite high to get good horsepower. I think you'd be looking for a cam package for a twin Holley 1946 or Carter Y series carb that would have to rev to about 5800 or 6000 rpm just to get 250 hp. That will mean a custom built engine.
For 350 hp using the biggest 1-bbls made, you'd be looking at 8000 rpm, which is not realistic.
