There are a few other things that will allow a powertrain to have a louder than usual "clunk" when going into gear. The high idle speed in Neutral/Park is going to contribute to the problem. There should be a drop in RPM when placing an idling engine into a forward or reverse gear. A 400 RPM drop sounds a little high to me. I am thinuing you may be running at too lean an idle air/fuel mixture. That may be caused by an intake manifold vacuum leak. There are a few different ways to test for a vacuum leak. One of those ways is to use a smoke test, which will help show exactly where any intake leaks are occurring. The one video I like is at:
https://www.youtube.com/watch?v=TmV_HjeiuCc
If you find any intake manifold leaks, correct them before trying to adjust the idle speed and idle air fuel mixture settings. I have some videos I posted showing how to adjust the carburetor idle speed, and the fast idle speed when the choke is (partially) on. It does not sound like your idle speed has an issue beyond it being set to a fairly high speed in Neutral to help prevent stalling the engine when it is put into gear. But, after checking for and correcting any intake manifold problems getting the carb set up correctly is important. The carb I am showing in my videos listed below is a stock Autolite 2100 2v unit. Your carb will be set up in a different manner, but the concepts are the same. Set the high speed idle first, then the running idle (with the TPS if so equipped), then the base idle (with the TPS off if so equipped), and idle mixture screws. I also suggest you set the base ignition timing before dealing with the carb idle settings. I cover that in one or more of the videos in the links below.
https://youtu.be/mwLwwUtI1cE
The link below includes a rebuild of the carb before the adjustment. kip forward in the video to get to the adjustment section.
https://youtu.be/U-ETjhRqUwQ
After the carb is adjusted,, if you are still running at high RPM in Neutral the clunking will likely remain an issue. Here are some other areas you can check...
It was suggested above to check the engine mounts. That is not unreasonable, but I do not see engine mounts cause a problem util there is some power applied to the engine and the engine ends of rocking out of its settled position, at which point it will make a significant sound. But, if the rubber insulation of an engine mounts has been eroded away, it is possible any engine noise will be transmitted with a metal to metal contact. In that eventuality I would concur an engine mount that is failed could make a clunking sound more pronounced.
There is also a transmission tailshaft mount that could possibly contribute to a slunking sound. But, like the engine mounts, usually I see those causing problems on a heavy load, not at idle speeds. But, as with the engine mounts that is no reason to overlook it as a (partial) cause of a pronounced klunk.
If either U-Joint is seriously worn putting the tranny into to gear csn certainly cause a loud clunking ound. It is easy to check a U-Joint for excess wear to the point where a loud clunking sound is heard. If you do not know what to look for there are a lot of videos in YouTube on the subject. Just search in YouTube on, "check for bad u-joint" and a lot of videos pop up. I have not read or watched all of the videos, but I have come across many that are reasonable. For me, rather than tearing things apart to inspect the U-Joint, I would prefer to forst trying to rotate the driveshaft in different rotational directions when the wheels are on the ground to see if you get any excess movement with either U-Joint. If you see excess movement, or hear any noise from the U-Joint(s) it is time to replace the offending part. If you have a new U-Joint being installed, be sure to pack it with fresh grease so it is properly lubricated. If you are going to use the existing U-Joints I suggest lubricating them with fresh grease also to both provide proper lubrication, and to help soften any sound they may be contributing to the clunk you re hearing.
While trying to rotate the driveshaft back nd forth you my feel or hear domr clunking coming from the driveshaft front you where is slides onto the transmission output shaft. It is very unlinkly the toke and output shaft splines have worn to the point where you hear a clunk from there or see/feel/hear the impact of highly worn splines. But, you can help reduce or minimize any clunking when going into gear by unbolting the rear U-Joint from the rear axle pinion shaft, then sliding the front yoke out of the transmission, Then pack some wheel bearing great into the yoke and output shaft so when you reinstall the yoke the grease is spread out between the roke and output shaft, You may need to indsert and remove the yoke a few times to mke certin the grease is applied along the length of the yoke, and between all sliding spline points. When reassembling the driveshaft make certain the rear U-Joint retaining U-Bolts are properly tightened. There is no need to over-tighten the U-Bolt nuts.
Yet another area a clunk could be coming from is the rear axle pinion gear when it is slammed against the ring gear when going into gear. If the rear differential has enough wear to allow a clunking sound, you will likely have other issues with the rear axle gear set. Those rear axle gear sets are very durable, and do not wear much as long as there is adequate lubrication (using the correct rear axle lubricant) in the rear differential system. So, I really dount you will have excessive play when manually rotating the pinion input in both directions back and forth. If you are getting excessive movement and/or sounds from the rear axle gears you will need to rebuild or replace the differential gear set. I sincerely doubt that will be the case, but it is just another stome to be overturned and examined.
I know this was a long read. But I am hopeful it is helpful. If the clunk continues, or is corrected, please post the information so all of us find out how you ended up dealing with it. I am going to be doing some acoustical waveform testing on a series of driveshafts and U-Joints over the next few weeks. If I come scross any other ways to test U-Joints for excess noise/wear I will be posting it in this forum. That may happen a while after the testing processes are being worked out, which may extend until Car Season 2024 if the testing and recordings of various U-Joints is not done before Winter Hibernation 2023 - 2024 up here in Snow Country (outside Rochester, NY.
