Where did you find the reference to the air distributor in WIS? I see it in EPC, but not in the WIS...I have WIS version 220.127.116.11.
In addtion, it is only the 1999 Viggens that have the two piece manifold. I checked my 2000 Viggen, sure enough a one piece. I then check EPC....2000 Viggen intake manifold model number is 91-92-592, which is the same manifold listed in the B20x section. Primary manifold in 1999 for the viggen was 91-86-107 (ie the primary of the 2 piece manifold). The '01-'02 Viggens use 93-99-528 which is essentially the same as the '00, except is drilled for the different pressure sensor used begining 2001.
With all this I got to thinking that maybe the reason the 1999 viggens have only 225 hp compare to the 230 hp on the '00 and up is due to the different intake manifold design....this wouldn't suprise me as I know mind flow improvements to intake manifold can yield 5hp easily. That two piece I bet disrupts the air flow due to the additional gasket...gaskets normally aren't a perfect cut.