If the other half of the ujoint is good, the eyes can get elongated, it's OK to replace just the front yolk. If you do not disassemble the sliding joint balance isn't a problem. If disassembled, correct assembly can be found in this article. One last comment, if you don't have a heavy duty vise and have never replaced a ujoint you may want to let a repair shop replace it. Look for a truck drive line shop or a 4 x 4 shop. Replacing a ujoint isn't always easy.
http://www.custompistols.com/cars/ar...ts_ujoints.htm
Clifton