dual coils

hi i am looking to buy a 73 duster with a 71 340 engine . the questions i have are the car has two coils hooked up one is a dui pak that goes to distributer the other is a accel coil that goes to a skinny blue box i have no idea why , also the car has a 8 3/4 rear with 3.55 posi it was a 4 speed originaly it now has a beefed up 904 will this hold up. i have the 4 speed it still has the pedal in the car what else do i need to change trannys and are the drive shafts the same thanks jim