1964 plymouth valiant Slant six to 340

You could use your original driveshaft, but it would have to be shortened. Good driveshaft shop could do that easily enough. Rear u-joint may or may not fit the rear axle, depending on the size of its u-joint. That can be addressed either by changing the rear axle yoke or by using a combination u-joint.

If your shaft is in good condition, it would probably be strong enough, but it's (I think) on the small side (2.5 inch diameter). Also, if your ball and trunnion joint is worn, the parts to rebuild them are getting more scarce and more expensive all the time.

There are companies that can build you a slip joint driveshaft that will bolt up to your ball and trunnion style transmission. I did this for my 59 Dodge. See Driveshaft Specialist Home I'm sure there are others.