Off the shelf 600's do ok on a 2x4 setup.
There are some things you can do to make them work better.... you have to remember the signal is now split, so the carbs aren't acting perfectly normal like they would in a 4v setup.
My biggest issue with "off the shelf" brand new carbs is that many of the low buck options are shoddily built. Often set with an idle too lean for a performance car, and often misdrilled holes or variability in the calibration.
Not always an issue, but sometimes it is
This isn't a problem with older built units, where quality was obviously more important.
1850-2's are cheap and easy to find, if you find some that look good, that would be a decent way to go, just rebuild them and slap them on.
I don't want to seem like I'm advertising, but the real benefit of dropping me a line is that I change some things in the calibration, but more importantly I can test them out and get a really decent baseline tune for you so you aren't trying to get them set right on a brand new engine.
If you are willing to spend a few hundred dollars on drills, taps, set screws, etc I'd be happy to walk you through getting the carbs set for your application, but if you are running a SFT cam and a new engine that can be a real butt clencher as 2x4's setups can be real touchy if anything is off just a little bit.
Good luck,
Dp