Want to put a magnum 5.9 in my 70 dart.

BUT-
Be aware of the difference in the 5.9's balance.
It takes it's own flexplate or flywheel, which in general does not interchange with any other LA engine (or the 5.2 Magnum either, for that matter).
Much of the rest will depend on whether you decide to run the Stock Magnum's EFI, or convert to carburetion.
"Least amount of cutting and welding" will depend on what your Dart is currently equipped with.