Depends on what heads the Mag has on it and what flywheel it has now.
Since the Mag has a matching 904 with it then either it has a Mag balanced flex plate already, or an balanced converter.
If it has the balanced flex plate then you can slap your 727 right on it (as long as the bolt holes for the converter are the same, and there are no weights on the converter.
If it has stock Magnum heads you will want to change those for something that isn't or doesn't crack, like EQ's and if you do that you will want to swap the cam, springs, retainers and clips for something capable of using the new cam.
You would be ahead by talking to our member magnummopar as he does this for a living and can give you some good details.
If the Mag was rebuilt by someone reputable with proof and already has aftermarket heads on it 1,400 is a total steal even without the trans.
