65 Dart 273 to 340 ?

The 340 will need different engine mounts. It will bolt up to the 904 AT, but the 904 will need to be built to handle the 340. Otherwise, the 340 block is the same dimension and bolts in the same way the 273 does.

Two thoughts, which are just thoughts:

1) 340's aren't exactly easy to find, and a 360 is cheaper to find and will make more power.

2) the 273 is a pretty unique engine, and is worth keeping on that alone.

That said, its your car, and you need to be happy with it.