Well it depends on how much port work KC did on the heads, but I would add 10 to 15 onto the duration. If the springs can take it, a bit more lift would help, too. A little chop to the idle, but far from radical.
I wasn't a fan of the 347, same as you. I have owned one for 4 years now. I can say in theory I like the 331 stroke better, but in reality there is nothing wrong with the 347 stroke. If your mind is made up, that's fine. It's your engine.
I still like the 4.125" bore blocks. Much stronger and better
oil paths. Although you should be fine with a factory block, with what you are building. It would help with your power goals. I don't see a 331 that mild making 400 lb-ft at the wheels, flywheel maybe. I doubt 270 Hp at the wheels, but it might.