Those chokes get sticky after many years. I've NEVER had to replace one in any of my Rochester carbed vehicles (and I've had many over the years). A liberal application of Gumout carb cleaner always got them working properly.
Oh, and check the A/C idle solenoid to make sure that's not stuck engaged...
EDIT: Other things to check that may have gotten screwy in the tranny swap...
Throttle cable adjustment
Cruise control actuator adjustment (There should be an ever so slight amount of slack in this chain (although its a cable on some vehicles, and a linkage on others, but usually a chain) with the choke and A/C off, engine warm.
Also, if the engine is running rich, and there's a vacuum leak, it will idle abnormally high.
I think my choke is stuck on, or maladjusted. It idles way too fast even after driving 20 miles. It's got a brand new pull-off but I guess that's not good enough. I have to research how the choke system works...I think I'd rather just have a manual choke.
Is the butterfly not open all the way when the engine is hot?
There is a fast idle cam that usually connected to the choke mechanism that bumps up the idle (keeps the throttle butterfly from closing all of the way) when the engine is cold- maybe this linkage isn't releasing properly as the choke butterfly opens.
Some carbs had a fast idle (cold idle) adjustment screw as well as a normal idle speed adjustment screw.
You should also have idle mixture adjustment screws that you can use to fine tune the idle mixture.
In my opinion, manual chokes can be a bit more fussy when cold than auto chokes- you just have to remember to fiddle with them/bump them open for the first few minutes as the engine warms up.
You could likely rob a throttle control from an old push mower and rig one up on a trial basis to see how you liked it.
I'll have to get a good look at the butterfly, and everything else, if I ever get a chance while it's not dark out. Stupid short days.
There is a label on the car saying that idle speed is not adjustable due to emissions. Research indicates that the screws are absent or epoxied. I should try to get a better look at it myself but I don't know carburetors and don't know where to look for them.
I like the mower throttle control idea for if I am going to try manual choke. I was trying to figure out how to do it with a bicycle derailer cable and shifter or brake handle but the mower stuff may work better. I have one that can donate its cable and lever.
The mixture adjustment screws are set behind metal plugs which are installed after its set at the factory. There is no easy way to get them out to adjust it without ruining the carb. Remember, this is the last year before computer control. Buick started computer controlling their V-6's in 1981.
With the non-adjustable carb, idling higher than 650 tells me that it's not actually idling at all. That's the problem I want to work on. Then if it's necessary to bring the idle up, I guess I'd have to hack the choke or the A/C throttle actuator solenoid (whatever that thing is called). Basically, I'm looking to cure a failure, followed by replacing it with a hack if necessary.
Realistically if it continues behaving with this 800-900 RPM idle I probably won't feel motivated enough to mess with it. This morning it behaved almost decently again.
However, first I have to get a tachometer on there. My biotachometer ears used to be very accurate but they could be maladjusted from disuse.