Replacing those gears requires patience, attention to detail and a little skill. Read and understand the procedure in the shop manual before you begin. As Fred says, you will need a couple of special tools, don't try to "ballpark" anything..
Bill M 53 GMC Hydra-Matic
Home Page http://home.comcast.net/~bd97/index.html
When you argue with a fool, two fools are arguing..