If the choke is functioning properly, it should idle around 1700 rpm when first started (fast idle cam). When the engine is started and the choke pull-off sees engine vacuum, blipping the throttle should bring the idle down to about 900 rpm until the choke spring sees some engine heat from the choke heater tubes. Then, the idle should drop to whatever you have it set to with the idle stop screw. With a fairly mild cam on a 421, idle should be around 700 rpm.
It sounds like your choke may not be closing when the engine is cold, so idle mixture is a too lean. After the engine warms up, and can run on a leaner mixture than when cold, idle speed increases. I would expect this would also result in backfiring and poor throttle response when the engine is cold.
There may be a vacuum leak complicating the problem as well, but confirming whether you have an operational choke would be the first step in troubleshooting.
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