Can't scan images, so that makes it hard.
It'll be very similar to what's on the bike in the picture except for the top chamber.
He's used a 90 degree pre-fab bend for the feeder to the rear throttle body and the same for the other 2 as well, but they have been cut down to blend in with the chamber above them all.
It appears the chamber is about 3" and straight thru till just before the last elbow at which point it necks down to suit the final feeder pipe size.
What I'm wanting to do is start out with a larger diameter and taper that chamber from 3 1/2" down to 2 1/4" over about 9". Not stepped in segments, but cone shaped. Does that make sense.
Effectively it will have enough air coming in thru the front to feed all 3 throttle bodies but progressively reduce along it lines as it goes.
A throttle body measures as being 56mm inside and 62mm outside.
2 1/4" is 57.15 where as 2 1/2" is 63.5mm.
As it's the 2 inside measurements I'm trying to align then I've gone for the smaller of the 2 sizes.
To get the correct surface area of the 3 feeder pipes combined then ideally it should start out at 3 3/4" inlet, but I reckon 3 1/2" is near enough.
So now, if I have got you totally confused you'll have to wait till I actually do the job and I'll start a thread on it with pics as I go.