not the header but you should be able to use the exhuast if the diameter is good from the start, if you plan on going through with theswap get 2.25 or 2.5 ID piping so that you wont have to redo it later on. 3" if your going turbo.
It's called header cause you only have one engine head not two, its not a V-engine. The header won't have the same bolt-up pattern when you switch engines but you could try to part out parts from your old engine and save the money for parts for your new engine. The GA16DE seems to have the same bolt-up pattern as the QG18DE but don't quote me on it since the chassis is different and you may run into problems when you try to install the downpipe. I remember I ran into clearance problems installing the hotshot header which supposedly was a perfect fit (you need to hammer the oil pan).