B18* family engines (B18A/B/C, B16B) have a 9mm taller deck, as in the actual engine block is taller, then the B16* family engines (B16A, B17A.) This is why B18* guts can't be straight swapped into B16A pattern blocks.

The exhaust manifold bolt pattern is identical, same flange.

The header will most likely rub the oilpan of the B18C, and will also not be the correct length for the rest of the exhaust.