RN wb and thru hull exhaust are bigger than SN/FX1. I've had good luck finding couplers on summitracing.com, we have been using the vibrant performance ones in a friends FX1 to run a type 4 with a stock rn wb. The ones I've used seem to be better quality than the factory b couplers.
http://www.summitracing.com/search/...part-type/tubing-couplers?GroupBy=ProductName
http://www.summitracing.com/search/...part-type/tubing-couplers?GroupBy=ProductName