Cam, you are correct. A 390 cfm carb is a better choice than a 500 cfm 2-bbl period. But Circumstances alter cases.....
390 PROS:-
A 500 is really a 354 cfm carb, and flows less air than a 390 cfm carb. So a 500's its potentialhp is less than a 390.
1.To set up a 390, you need the right intake manifold. The US guys with there mighty 300's and 240's have it all over us Aunzies here. A vast array of manifolds exist there.
2.It's jetted lean for about 150 hp. A good cross-flow 4.1/250 will need a lot more than that as the stock carb restricts it so much. 390 4-bbls are jetted leaner than most 500 2-bls.
3.The other issue is the emmissions. In the US, a 390 is easier to get past a sniffer test because they come with proper evaporative emissions and reverse idle jetting. Some replacements have fine jetting which allows a new carb to be set up to pass the CO and HC emmisions very easily. Holley has listed the 390 cfm as a emissions carb upgrade. It's technically illegal though. The rule is, if it didn't come out with a 4-bbl, its not legal, as it has no Executive Order or California Air Resources Board sticker. Most emmissions Nazis are happy to pass one if it doesn't fail the visual inspection, though.
4.Additionally, the 390 has a 195 cfm primary which is much smaller than the 354 cfm a 500 Holley really puts out. So it can lean cruise much better, and still out perform a 500 if its set up correct.
5.Upshot of all this is balanceing off the space verses performance verses economy. If gas mileage is king, get a 390. If performance is king, get the 390, and jet it up more. If space is at a premium, get a 500.
6.If the Ford motor company decided (in the past) to build hot six cylinder cars, then they would have gone for there Autolite 4-bbl carb with an alloy intake much like the cross-flow Ultraflow manifold, and used a carb more like the 390 cfm. But then, who would buy a 2-bbl 302 if there was a 4-bbl 250 or 300 lurking around? It would have canned it!
500 PRO'S:-
1. A 2-bbl is easier to set up for an Aussie cross-flow. There's not much space between the spring towers for XD to XF Falcon, less in the XA-XC's, and less again on the XR to XY's. And none at all in an XK to XP. So a 390 wont fit a 250 /4.1 easy. Nor a 250 log. This is not an issue with a 300 in a f150 truck.
2. The set-up of the secondaries, and jetting is a little harder with the 390 verses the #2300 series 500.
3. Guite a few 500 cfm carbs are set up for 327 to 400 cube V8's with 180 hp capabilities. A 2-bbl will always work better right out of the box.
4. A 500 2-bbl is not an emmissions carb, and has been designed as a pre US 1973 era '49 states' carb. That means in areas not subject to annual visual or sniffer inspections can use it. In California, a 500 is an off road carb only.
5. The legal use of the 500 cfm carb on emmission era cars is an issue for the US, not Aussie because most of your inspections deal with whether or not the original emmisions stuff is on the car.
6.The 500 (#2300) is also just a simultaneous opening carb, with both throttles locked together. It is easier to set up.
