The main priority for jetting is cam profile. A bigger cam requires bigger jets, because it has less vacuum for each intake stroke, due to the overlap. But engine size has comparatively little effect, because if two engines of different sizes have similar cam characteristics, they still need the same amount of fuel relative to the vacuum signal at the carb, it's just that the bigger engine is sucking it in faster. That said, if your engine is relatively stock, and that carb was set up for a stock 390, it should run about right on your engine. The standard jets and calibration are the best place to start.
The thing to remember about carb jetting is that it changes the fuel flow, not the airflow.... "jetting down" for a smaller engine is a great way to melt pistons, because you're cutting fuel, when, in fact, the smaller engine is pulling on the carb in a similar way, just not as quickly, if that makes sense.
With all of that said, the only real control you have over airflow is the vac secondary spring, and you could put a stiffer spring in, but there's no real reason to, as the 300 will have a tough time even getting the secondaries fully open on a 600 with a yellow spring. For tuning the sec. spring, I just start with the lightest and work my way up until there's no bog... if mileage is a very important concern, use one spring stiffer still... but that should never be an issue, unless you regularly cruise at WOT above 3000 rpm.
edit: ps, I'm drunk. Even so, the point I was getting at is that carbs need to be set up to run with cams, not engine size... they need to be CHOSEN based on engine size and rpm range (aka total airflow requirement), but the point I'll reiterate again is that regardless of whether a carb is too big or small for a given engine, the optimal setup will be VERY similar on a similarly built engine, but of a different size. Remember that, and you'll be fine.
