Sniper,
The oil holes on the left and right sides of the head won't line up without some metal work, and you'll need to swap out some of the engine studs to shorter ones. The cylinder diameters are different too, so the combustion dome will overlap the cylinder walls and you might need a custome cylinder head gasket. Compression ratio will also be slightly decreased. For these reasons, if you are going to swap a 400 head onto a 350 engine, then swap the cylinders too. I think that will require opening up the holes in the upper case on the 350f to accept the larger 400 cylinder liners though... again... a headache. A complete engine swap would be easier.
Try porting out the 350f head instead. It'll provide a lot more power than the stock head and won't create all the fitment issues of force fitting a 400 head and cylinders to your 350 block. The big difference between the two heads is better flow on the 400f and larger intake valves. You can easily fix the airflow issue by porting the 350f head. The displacement of the 350f doesn't really justify the larger intake valves for street use though. You'd need to run higher revs to really benefit from increasing the intake valve size, and you probably don't drive around like that anyway. If you are looking at power, then stick to the porting for a quick power increase in the streetable RPM range.
One of the early posts in this thread referenced another thread where a bunch of us commented on how to make more power out of these engines. We all seemed to be in agreement on how to do it, so you'd do well to read that post.
Camelman
