904 to 727 Swap

I am working on my first Mopar build a 1967 dart for my son and do not have a lot of knowledge on what parts interchange. I am going through everything since the car has been sitting a long time. The car currently has an A-904 in it but I have a opportunity to purchase a small block A-727 that has already been rebuilt for a good price. The engine is a 1970 340 making around 400hp. I am just not sure if I will have to change a bunch of stuff that might not make it worth the money saved.
I appreciate any info I can get.

Thanks