You will not have any problem with the 600 cfm carb on your motor. Its a 300 cid motor. when you run the calcs it will show the carb too big. But I have ran 600 and larger on many 290-350 v8 cid motors. Without having to do any sort of odd tuning due to it being too large. In general, too large of carb will not provide a good booster draw, forcing you to jet it richer, whcih in turn causes other problems. I have never had to jet any more than 1-2 sizes differently. even ran a 289 at one point with a 700 double pumper that actually needed to be jetted down on a couple of the holes, indicating very good high velocity inlet draw.
On my 300 six. I am currently running a clifford with a 625 AFB and have had no tuning or running issues whatsoever. When i was running a Offy DP, I would swap between a holley 1850 (600 vac sec) and the same AFB without any issues (once I found the source for the inconsistent vacuum leak). I ended up using the AFB becasue to me it simply looked better than the holley, oriented the way the DP carb is. I orinted the top on the clifford the same way. With the carb facing the cylinder bank, it should be getting better charge distribution. I am getting right around 15 mpg for city and country driving.
running a smaller carb is a plus in that there is some opportunity for economy and response. But the tunability, drivability, etc with the 600 cfm carbs on a 300 cid is not the issue that everyone seems to make it out to be. The carb should bolt right on with the factory jetting and prep and run just fine. the accelerator pump tuning may need some tweaking for perfection.
Many times I have been seeing all these problems on the carbs. Most issues of carbs are from buying a carb that someone else "tweaked" (albeit wrongly, usually). even recently on this board, such a story occurred.
I did a cam for a 10 second combination, in which the motor simply did not perform to expectation (10.90's at best). they insisted on 5-7 sizes richer (it was a race car after all!) regardless of what the A/F said. they never could get it right, and it was OBVIOUSLY the cam's fault. The motor was sold and the new owner bolted his intake/carb/headers and his vehicle ran 10.20's (similar weight) and both capable drag set-ups. MAybe it wasn't the combination afterall, but simply the tuning.
It doesn't run quick at the track, but it gets long stares wherever it goes.
Never in a million years did I think I would have an antique hot rod truck, much less with its own name emblazoned on the front fender and a freakin' six cylinder for power;
but it is a Ford, it is old, and it is definitely one of the funnest vehicles I have driven.