I found an old thread about a 1978, pretty much the same design as 1974 model.  Here's the first post and a snip from the follow up 

Hello I have a 1978 DT250 I'm restoring. I need to replace the clutch arm that actuates the push rod because the part where the cable attatches is all beat up. In the manual it says to just loosen the lock screw by the front sproket and it should pull right out. Well mine isn't. It stops about half way  when trying to pull it out. Does anyone have any experience with this? Thanks. 

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So I took off the right case cover off and just removed the bolts and springs. After that I pulled up on the clutch arm and it slid out. When it slid out, the clutch plates moved out a bit so yeah the push rod wasn't allowing the arm to be removed. ......


Just something to keep in mind in case it doesn't slide out...  hopefully it will like RT325 and the guy in the video say though!!!!

We'll agree to disagree Mark until the result is known but because it tapers 'up' you can't pull it straight down. But 'i think' it strikes the pushrod "off center" on the taper.
Off center so it can be turned away enough to extract it.
"I think" it's desighned that way because if it hit the paper dead center it would just roll around the end of the pushrod & do nothing.
I'm sure Yamaha wouldn't be 'that' difficult haha [would they--maybe they would]. My thoughts for the day.
Found a pic i'll link but doubt it shows what i mean.
photos.app.goo.gl/zVdZv8cnhLBNpopm7
Sorry, i shouldn't keep on keeping on lol. I'm outa here.

I just did this operation about a yr ago to my 74DT250A
All of this stuff Mark and RT are saying is spot on.
I did not have to remove the right side case as has been suggested, but am not going to rule it out as wear could be a factor.
If I recall correctly the arm dropped out quite easily after removal of said screw under the sprocket cover. It's a fairly easy task, good luck

I understand what you're saying RT325 and I hope you're right!!!   Lots of extra work to pull cover. 

The only one I remember doing without having the engine torn down was a 175 and I had to relieve the pressure on the pushrod to pull it up and out, rotating didn't seem to work. 

Here's a random could of pics from ebay showing how the actuator vertical shaft is offset from the trans main shaft, which i guess it has to be to swing & push if that makes sense..
Sorry to keep this going. Was just googling being nosey & thought ok, link it.
photos.app.goo.gl/Uuh9WCHjWNGffQWH7
photos.app.goo.gl/3uNon91dYg5XzMZj9

Thanks again guys, That is the exact video I ran across that started me thinking I can replace the seal without splitting cases. I will definitely provide feedback and outcome. And I will try to video the process. You guys really provide great support. I appreciate it.
Sincerely

Thanks guys. that is the exact video that started me thinking I could replace the seal without splitting the cases. I will provide feedback and outcome when complete. I will also try to video the process. You guys really provide great support. I appreciate it.
Sincerely

Good Morning and Happy Holidays, I did replace the clutch shaft seal in my 1974 DT250. Very simple. Drained oil, disconnected the clutch cable at clutch arm, loosened clutch adjusting nut and backed out adjusting screw until the shaft slid right out. Plucked existing seal out with a screw driver. Installed new seal and reversed process. Not a drip since.There are a couple things to be aware of during the process........1. Take a picture with your phone of the position of the arm and especially the position of the return spring on the clutch arm.2. Use a sharpie or other method to mark existing depth of the clutch adjusting screw before backing out. (I simply. loosened the clutch adjusting screw nut just enough to begin backing the screw out. So when I reinstalled it, I could turn the screw until nut made contact. Then made final adjustment because I was already close to proper setting.) Keep in mind it is necessary to confirm screw is engaged properly with clutch shaft when reinstalled.3. There is a washer that goes in after new seal is installed before shaft is installed.4. Revisit your picture of the return spring and reset to original position before installing the clutch shaft. Then set it back to original position for tension. Good Morning and Happy Holidays,I did replace the clutch shaft seal in my 1974 DT250. Very simple. Drained oil, disconnected the clutch cable at clutch arm, loosened clutch adjusting nut and backed out adjusting screw until the shaft slid right out. Plucked existing seal out with a screw driver. Installed new seal and reversed process. Not a drip since.There are a couple things to be aware of during the process........1. Take a picture with your phone of the position of the arm and especially the position of the return spring on the clutch arm.2. Use a sharpie or other method to mark existing depth of the clutch adjusting screw before backing out. (I simply. loosened the clutch adjusting screw nut just enough to begin backing the screw out. So when I reinstalled it, I could turn the screw until nut made contact. Then made final adjustment because I was already close to proper setting.) Keep in mind it is necessary to confirm screw is engaged properly with clutch shaft when reinstalled.3. There is a washer that goes in after new seal is installed before shaft is installed.4. Revisit your picture of the return spring and reset to original position before installing the clutch shaft. Then set it back to original position for tension.

Sorry about the double content. I don't know what happened. I also tried to add pictures in editor but it did not like me or I did something wrong. I wanted to share.
Anyway, Thank you all for your input and support. I am grateful,
Smooty

Congrats   Thanks for sharing 
​​​​​​ You can upload pics via Imgur or google photos