Swapping rearends, need your input.

You guys have been really helpful to me so far, and it it MUCH appreciated!!!

Here's another one for you.

I have a 74 duster with an 8 1/4 and a 904.
I have a 71 dart with an 7 1/4 and a 727. (po didn't finish)

I want to put the 7 1/4 in the duster. It has a 318 and will be a daily driver.

I want to put the 8 1/4 in the dart. It has a 340 and will be a Street/strip car.

Is this a direct swap?
Will I be having any driveshaft issues?
Is there anything I need to know before attempting this?