Brian,
The carbs that came on a medium rise 427 were dual 715 cfm Holley 4160's. Your 600 cfm carbs are fine. What you need to first understand is that you will never come close to using 100% of their capacity of 1200 cfm. Just know you will only ever draw what your engine needs and no more.
Most FE's will draw 700-900 cfm. In the real world that means a 750 or 850 4150 double pumper or even a 4160 vacuum secondary 750. I run dual 450's for regular street use. They are 4160's but are mechanical secondary units.
Since you've decided to do this yourself know that anything you do wrong is easily fixed, nothing you can do to a Holley carb will damage your engine (aside from fire caused by sloppy work) and the worst you can do is learn something. Some things more than once.
The first thing you will need is a new set of vacuum secondary springs. You may want to start with the lightest springs and work your way up. You will notice a big difference in throttle response with a light verses a heavy spring, obviously whatever you do to one you need to do to both so be sure to get 2 of everything.
You know when you've gone too light with the springs when you get off idle,say 2200-2500 rpm and you are taking off fine and the engine suddenly bogs. If it doesn't actually bog but you get a belch of smoke you have also gone too far and may want to use the next heavier spring.
All 1850 Holley Carbs are equipped with a power enrichment circut called a Power Valve. Power Valves are like a shot of fuel controlled by the amount of vacuum in the low pressure side of the venturi. Once the intake vacuum drops to a predetermined value, in this case 8.5 inches (Hg) the valve opens.
Here's the thing, many mildly built FE's don't ever see 8.5" of vacuum. My 427 idles at 5.0-5.5" with the 450's. So you will first need to get a vacuum reading off the engine warm at idle. If the idle vacuum is over about 11.5-12" the standard power valves should be fine.
If your engine idles at 10" or less you will want to change the power valves to a lower number, generally 2.5-3.0" less than that of the normal idle value. I have 2.5's on my 450's
Assuming the carbs have not been molested, or significantly modified standard jetting will be fine for your 428. You can if you find at mid throttle you are belching fuel smoke or running rich rejet the 600's to something a bit leaner. You simply pull the rear (because your carbs are backwards) float bowl and remove the metering block and install new jets.
The 600's you are using are very close to the carbs Ford used in the 60's. Ford got over overcarburation issues in part by using a progressive linkage. That is the primary or front carb did most of the work, and depending on the way it was adjusted may have opened 30-50% before the second carb kicked in, often less.
I am from the school of whatever the front carb is doing the rear carb must also do. This is critical in a single plane performance manifold world or a multi carb setup. It isn't really important using a standard Ford dual quad manifold, nearly all of which were dual plane. This is part of the reason I opted for 450's. But I do the same with a pair of 660 cfm carbs I use from time to time. For drivability you may find adjusting the linkage to a given point for daily driving or cruising, and another point for better performance.
I don't have any photo's but I use to race a 12a Mazda rotary (1200cc) that employed a 1850 Holley. I was too poor or too stupid to realize they made a smaller carb. I'm sure the one I ran was free or nearly so from a swap meet. It drove and ran very well after a good deal of sorting.
Whatever you do only perform one task at a time, by that I mean don't go making two, three or four major changes off the bat. Start with taking a baseline vacuum reading. If the vacuum is in line with the stock power valves move on to another task. If not select a power valve and change them out, but don't make any further changes until you test run the engine. This will allow you you to undo anything you did without wondering what went wrong.
We'll be here to pick up the pieces if things get really bad. One thing you should always remember. Most "carb" problems are really fuel delivery problems. Stale fule, water in the fuel or dirt in the fuel or filter. One thing a Holley (or any) carb needs is clean fuel. If the idle circut air bleeds or the cruising circut air bleeds get dirty nothing will ever work right.