I was checking transmission prices today and happened to bring up the k20 350 to 400 tranny swap (I have a 1976). I didn't take notes (kicking myself now, because I'm considering the swap too) but the guy said is that the 400 is about 4 inches longer than the 350. Also, I believe he said that the output shaft is a different size, which means you need an aftermarket adapter of some sort to connect to the transfer case, not to mention changing the lengths on the driveshafts to both the front and rear axles. I got the information from a guy at a transmission rebuilding shop. If you have a similar shop (all they do is rebuild transmissions, all day, every day) in your town, give them a call. Those kinds of places usually have at least one guy that can give you the info you need.