74 dodge dart tranny issues

I just got my 74 dodge dart custom. It has a 318 in it and a 3 speed auto trans. Sometimes when your driving it, it slips out of gear It does it worse when its cold. Fluid is full and looks good. Any suggestions or does it just need a new tranny? also is a 4 speed auto trans an easy swap. If I have to replace, may as well upgrade. :)
Author: admin