I have a 65 Barracuda with a slant 6 - three on the tree want some transmission/engine advice

LOL...love the replies... I would prefer to keep the slanty, but I am not worried about numbers matching (its a factory slant). I had a /6 in another vehicle I drove over 300K so I have a bit of an affection for the engine, and spent some money in a previous life with clifford in another 6 banger (rambler american). Just want not sure how much work a 904 would be to swap (drive shaft, etc). I have done lots of auto work, just newer to most mopars...I appreciate the guidance.