1964 plymouth valiant Slant six to 340

Since a '64 never came with an 8 3/4, there are no factory ball and trunnion driveshafts that will just bolt in. There may be a few out there from cars that had an 8 3/4 installed at a later date though. If you have to cut it shorter and rebalance, you may as well upgrade to a better driveshaft so you don't have to worry about breaking a stock one. If it's a 741 case 3rd member, yeah, probably small u-joint, 742 or 489 case could have several options depending on what it came out of.