Typically on that size engine, a 600cfm Holley is the best all-around choice.
I wouldn't pay much attention to the cfm calculators, as they are VERY conservative. Also consider that despite the numbers the calculators give you, factories consistently put larger or smaller carburetors than the numbers showed were "right" on engines.....and they ran fine.
Often, you'd see engines that had two carb options...maybe a 2bbl on the base engine, and a much larger than a cfm calculator recommends 4bbl as an option.
To recommend one for you, I'd want to know how your motor is built, and what your wants/needs for it are.
For now, since you went to the trouble of upgrading to a V-8, I'm going to assume you desire more performance. In that case, the 470 isn't going to cut it for you. In the example someone listed above of a 470 on a 304, here is what you'd experience on stock 304's:
The 470 might feel a bit more responsive off idle, due to the smaller venturis compared to a 600. But, they 470 will quickly run out of breath as you rev it higher compared to the 600. That's not to say the 470 doesn't/wouldn't run just fine.....obviously, it will.
But let's say two identical Jeeps lined up side-by-side, and the only difference was the carb size. They take off, the 470 might get a nose ahead briefly, but the one with the 600 will walk off and leave it in short order.
Gas mileage with both properly-tuned is going to be a negligible difference. Remember, the 600 is still only 300cfm if you keep your foot out of the secondaries.
So, if it were me with your engine, I'd get the 670 Truck Avenger (too bad there isn't a 600), and just put a slightly different secondary spring to keep the back barrels from coming in right off the line...that'll take care of your throttle response, and you'll still have the benefit of the extra CFM that your engine absolutely CAN use at higher rpm.
If you don't plan on off-roading at steep angles, or if you're mechanically-inclined enough, get a 600 Holley and mod it to essentially be a Truck Avenger. Get the spring-loaded needles and seats, and install a rubber hose to connect the bowl vents with a vent hole in the top (old drag racing trick) and you have yourself a home-made truck avenger and the best size carb for your engine.
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