Re: Just a quickie
The 8v has a flange on the cat to middle section. The 16v is a sleeve fitting from cat to middle section, although I believe that some of the earlier 16v's had the flange type cat. Have a look at the cat fittings as this will be your biggest problem. I don't know if you could swap the cat over, but they are both flange fittings to the downpipes. The sizes and number of bolts may be different. The 8v has 3 bolts, for some reason I think the 16v has 2 bolts.
You can make anything fit with enough time/money and welding !!!!