I have a gti as my daily driver and should point out the stock manifold is crap. You should get 4-2-1 extractors made to suit a sierra conversion. They are available and should not cost too much but not too many people stock them. Try
http://www.suzisport.com/contact.asp or maybe put a post on
http://www.redlinegti.com/ to get it from someones stash. The engine is a g13b in gti's, mk1-mk3 should have the same exhaust ports but might have different bends for a fwd, not relevant for a sierra but there could be different designs.