You must have an original. That fitting is now threaded. They send the WRX and LGT adapter in every box. So you can install in either now, with no reducer. Yes, due to deeper core you usually have to trim the hoses slightly. Otherwise they kink.
All Mishimoto products come with a no questions asked lifetime warranty too. That includes crash protection. If your Mishimoto product is damaged in a crash just send in a copy of your original receipt and the police report from the crash and they send you a free replacement. Woot!