Do i need to take off the part that is onto the tranny too, or can i just plug the speedo into it with no issues? I pulled out my stock one and got another one...

 

You don't need to take it out. There is a clip holding the cable. You unclip, pull out, put in and clip again. It's simple. If it's rusted in you might need to replace the speedo assembly from a used tranny. That's what I had to do after I found a seized cable in mine.