Good 400+ HP 360 build

Well although a magnum would probably be the better choice I'm going to stick with an LA 360 because I already have a brand new top end. And I'm planning on replacing the pistons anyways, I just don't want to bore it out, I would prefer to keep the stock 360 bore and stroke. I'm starting to think that 400 hp without doing that may be a bit much, would around 350 be reasonable with the right tuning and cam then?

Totally doable. Listen to Mike the Mopar "Whisperer" from MRL. Solid flat tappet in an LA is a good choice especially If your going new pistons and can get the compression up a little. What new top end do you have? Stock heads reworked? Aluminum heads? Intake? Nicely prepped heads and an air gap or rpm intake and Holley or Quick Fuel 750'ish carb will get you there no prob with a good cam. Devil is in the details...