(1) If the orifice is OK, should be fine.
(2) The main discharge nozzle is not a normally replaceable part. It can be replaced, depending on the skill of the restorer, and the tools in the possession of the restorer. Yours should be OK.
(3) See below
(4) The original valve was spring-loaded, with a small orifice, They come in the better rebuilding kits.

(3) The original issue of the Carter YF's used a small check ball, topped with a cubical brass weight for the pump discharge valve. After a few years, Carter obsoleted the ball/weight in favor of a pointed square brass needle of equivalent weight. The reason, folks would turn the carb upside down, and lose the brass weight. The pointed square brass needle comes in most rebuilding kits, and supersedes the ball/weight.

The metering rod.

Carter began using metering rod technology in 1929, and is one of the MAJOR reasons to use Carter carburetors when possible.

Think of the metering rod/jet combination as an automatically adjustable main jet.

Here is a link to the article to which Jon G. referred: Carter YF 235

Check the chart in the link, and you will find the 2100 has a three step metering rod.

The metering rod is inserted THROUGH the main metering jet. The effective metering AT ANY VALUE OF VACUUM is the area of the jet less the dynamic area of the diameter of the rod then in the orifice of the jet.
So the effective area of the main metering circuit varies at three values of vacuum, whereas the effective metering on the Rochester B has only two [(1) the area of the main metering jet, and (2) the area of the main metering jet PLUS the area of the power valve].

One can see from the chart that the metering rod has three different diameters: (1) 0.067 at the highest vacuum, (2) 0.060 at a midrange vacuum point (without pulling the print, guessing 10~12 inches of vaccuum), and (3) 0.050 at wide-open-throttle.

Because of the extra step, it is possible to keep a more consistent A/F density over the full range of engine vacuum, which improves economy, sometimes significantly, assuming the metering rod is calibrated for the engine.

What kind of economy is possible? I no longer own a Chevrolet, so cannot give an exact answer, but my non-Chevrolet shop truck has 390 CID engine, and two 625 CFM Carter AFB's (with metering rods), and gets 22 MPG at 70 MPH.

Jon

Good carburetion is fuelish hot air
The most expensive carburetor is the wrong one you attempt to modify.
If you truly believe "one size fits all," try walking a mile in your spouse's shoes!
The Carburetor Shop