You can use the 300EX spindles, the hubs are the same between 300EX and 400EX. The 400ex spindles will bolt up but you got JB long travel arms for a 300EX, so I use the 300Ex spindles.
the tie rod mount is in a different location on the 400ex. the stock 300ex spindles will bind at fool droop sometimes depending on what a-arms you use. plus 400 ex are a lot cheaper to replace and lighter.