Is the rear of the 400 headlight intact? If so you can use it with the 600 front.
Makes it easier as the rear will have the holders to take the H4 bulbs. but you will need to drill a hole to take the parking light
If you use the 400 rear of the light unit, the standard H4 bulbs will need to have two of the location tabs cut off to fit the 400 holders.
If not you will have to modify the left holder of the 600 light unit to take a H4 Bulbs
Let me know what you are doing and I will go through the rest with you