Rabbit summed it up pretty well.

I think even with the notched oil pan you'd probably be fine running a longer header. You will loose a bit of clearance, but I still doubt it'll be the lowest thing hanging down. Thats been my experience with other cars at least. If it does have some funky bends towards the back of the engine bay that'll be the determining factor in length.

When I was looking for design info I couldn't find anything on 4-2-1 type design. I believe with the extra collectors you get better low end scavenging. At higher rpms, without a great collector design, they'll likely create turbulence and reduce power. Thats really just my best guess though. Beyond that I wouldn't know how to size them properly or what length to make the primaries and secondaries.

IMO Xaiogary's header Is too short. I'm sure its a little better than stock, but not nearly what it could be.