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Some things you need to find out:
the carb no. you gave seems to be what is generally known as a "4150" series, and is a 650 so called "double pumper" which means mechanical secondaries and two accellerator pumps.
Several problems plague all these series of carbs. One is that sometimes, due to age or backfire, the power valve can leak or seep in various degrees, resulting in rich, rough idle, up to the point of being nearly undriveable.
Also, sometimes the return linkage/ spring on the secondaries sticks barely open, a recurring problem on the so called "spread bore" designs with dual pumps. You will be going down the street and the throttle won't "close."
Also, those carbs used to come with a horrible mechanical choke. I usually just removed the choke, but that was in my Navy days in S Cal.
If the carb you have is jetted right, is operating right, it should run pretty good. You might need someone to rebuild it rather than replace it, and at high altitude, big cams, or other situations, it might just need to be set up differently.
Something you need to find out:
The original '71 340's should have had a so called "spread bore" design, fitting the horrid Carter Thermoquad (Thermojunk), or Holley "spread bore."
If it has an earlier factory manifold, it may take the Carter AVS
If it has an aftermarkit manifold it should thake either the standard Holley "square bore" or Carter AVS/ AFB
Back when I swapped the 340 into my '70 RR body I actually ran it for miles and miles with a GM quadrajet.
One carb I would consider, nowadays, are the Carter (Edlebrock) AFB series, which you can also get with electric choke You should be able to find a "dealer" to recommend a specific model for you
You might post some possiblilities you are considering (links)
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