Want to put a magnum 5.9 in my 70 dart.

Professor Fate said
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.

Yeah I ran into this with a magnum based motor recently. If he decides to go 904 you gotta pay $$$ for a flex plate. I tried the stock magnum flexplate but it wouldn’t fit my PTC converter. Something for him to keep in mind if he is going with an aftermarket torque converter.