904 or 727

Its been a while since on this site. Had to let my 69 BB valiant project go for financal reasons. But since have purchased an other ride its not a mopar but has mopar drive train. 1963 AMC Rambler 550 CC wagon. It has 318/904 setop in it. My question is how much different in size are the transmissions between 904 & 727. My 904 is slipping and found a 727 thinking about buying and going through and installing. WOuld like either healthy 360 or 5.7 Hemi down road. Thank you in advance and attached a picture of my new ride.