interchange

lets just say that plans are to pick up a 71 dart with a 400 b...
and just so happens that i have a 69 forged crank 440 rb....
should i interchange some parts and keep the 400 in the car and add 4 speed http://www.forabodiesonly.com/mopar/images/smilies/3gears.gif or build the 440 and and the gears....what kind of power am i looking at and around how much cash:evil4: all comment will be considered