Time to rebuild?
The rebuild won't happen until summer anyway, so I have time to try a few things first.
Yup. Summer's a much nicer time than winter to do heavy-duty car work.
Regarding the float needle: I assume if it were gone, the engine would be completely flooded and not running
That's correct.
is it possible that somehow the carb bowl could be overflowing occasionally?
Sure…and you'd know it. If it were happening after you shut off the engine and park the car, the whole area would reek of gasoline. If it were happening with the car running, the engine would gag and belch clouds of black smoke, if not stall.
my idle mixture screw has a plastic cap that prevents it from turning more than a half turn. Does that mean I shouldn't need to adjust it?
No, the adjustment is still very much needed — as described after the links to carburetor operation and repair manuals in
this post. Idle mixture limitation was mandated early on in the sequence of cleaning up car exhaust emissions. That doesn't necessarily mean the optimal adjustment is within the range allowed by the limiter cap. I routinely remove the caps (pry off with screwdriver and/or pliers) and then adjust the idle mixture optimally.
it starts right up but then after a few seconds starts to bog down and doesn't idle well until it gets warm. Once warm it's fine.
The '74 choke is factory-set on the lean side, and it is not adjustable. A № 1234
electric choke kit will drop right on and enable you to adjust the choke tension and duration to get rid of the mushy acceleration during warmup.
The PCV valve is correct: it's stamped 2103
That doesn't tell us anything.
is the one listed for a 1974 225
Listed where by whom?
I'm still holding out hope that my valve cover baffle is simply clogged with crud,
Would be nice, but that's probably not it. You'll see why not when you turn the valve cover upside down and look at how the baffle is configured.