Rods should be fine to 5500 or so. The thing is if you're going to use them they should be at least checked for stretch and new bolts installed which is going to cost almost as much as a new set of 5140 rods so why would you bother with the old ones. Just consider them like old bearings and buy new ones even if they look fine.
If you're setting your sights on 400ft lbs of torque you'd probably be better off just shelving the 350 and going with something bigger. You could spend alot of money on that 350 and never get where you want to be. You will pretty much spend the same money on a 455 to rebuild and you'll end up easily reaching your goal.
Just my .02