V8 conversion

hey, so you guys say its pretty simple to swap /6 to a 318? i have a 71 valiant 225, and i want to put in a 318 A block poly built from 1956-67. would this stil work even tho its a different era? would everthing line/match up? im thinking i may also want to get rid of the 3-spd auto(lame) and put in a 4 spd. would doing this make the task easier? anyone have a comment?