New to all of this

I have a 72 swinger with a 318 and a 904 trans. Today i bought a 1977 400 with a 727 trans on it. I have a question about the tranny. It has a short tail shaft on it. Can still be used or does that have to be replaced in order to work in my car? If so, How hard is it to swap out? My bro in law also has a swinger and a 440 rt charger he has an extra 727 with the long tail section on it. Is it pretty basic to swap them out or should just have his tranny rebuilt and use it. I am a recent convert from a 67 camaro, so a lot of this is all new to me. Thanks any info is appriciated. Also is the 400 a good engine or should i look for something else.