You will need to make a pattern of the port flange and collector, the collector block can be clamped to the chassis .Before you drill the collector block or header flange drill the centers out to take a piece of 3 or 4mm welding wire, coathanger or the neighbours fence is also suitable
. At this stage we will go anal and make the header an equal length jobbie so chop up the wire into whatever length wire it would take to get from number one port to the collector. Find a tin or somthing the same diameter as the rad of the bender and fashion the wire to form the header as near as you can to the finished item .Spend some time with the wire before you rush into the bending and you will be able to measure the wire to get the correct centers. Bear in mind that the true pipes will need to be say 40mm or whatever size you want and they dont want to be touching each other, also consider that you might want to wrap them. Its now time to drill the holes out to 15mm. They will look odd in the small pipe but when the real ones turn up they fill all the gaps as you can see down at the collector. Its also in the ideal world best to arrive at the collector in rotation of the firing order. If its not cammed up and running mental hp dont worry to much about the equal length pipe and collector order. Also remember to buy some straight joiners in case you f@@k up, I do quite often.
Here is this one in pipe but the design is down to you.
You can do what you like model building like 4 into 2 into one.
I prefer to have them knocked up in stainless because you can polish and forget but there is nothing wrong with mild steel and say ceramic coating. Will do the collectors next.
I hope all this is of some use .
Bob