Slant 6 to 340 swap

Hey I really need some help.
I have a 74 dart sport i'm converting to a 340 car. I have already got the motor mounts i need in order to swap out without changing the k memeber, i have the 727 tranny and the bigger 8 3/4 rear end. I am rebuilding the rear end now and the question i have about this rear end is that i was told i had to change the axles from a 5x4 bolt pattern to the big bolt pattern of a 5x4.5. obviously this will make it a little stronger, but i dont understand why i have to swap. If i put in the bigger bolt axles in the bigger rear end then i am just looking for more change in wheels and drums and brakes etc. I just need some direction in this it would be very much appreciated. do i have to change to bigger bolt pattern or will i be safe with the one thats already in it? Thanks! :)