'74 Dart V8 swap

I must apoligze. I thought the magnum and roller motors where one and the same.(I need to just stick with chevy motors). I say 340 just because thats what dads always wanted. Guess a 1972 duster 340 gave him a good whipping in his 71 ss 454 chevelle back in 73 or 74..... hes wanted one ever since. Yet he ends up with a slant 6 dart..... Makes no sense to me