Hi Enoch,
From memory, I'm pretty sure the cast manifold you have is the only one made for that engine.
The 16v head/manifold is totally different. It might be possible to use the 16v one if you cut the flange off and make one that suits the 8V head but thats a lot of biggering about in my opinion.
Make something to fit the stock 8V outlet - possibly adapt the original downpipe.
The 8V is not a high performance engine so it's not like it's gonna affect performance much using the stock manifold.
HTH BV
