I hope your homebuilt spacer works for you. I read a test where a guy put an FCR39 on his XR650L with an exhaust pipe adapter and it ran like crap. Google "Noss adapter". It attaches to the intake side of the carb and provides a smooth transition. The 37mm carb should have awesome low end snap.
Harley, been there done that. Don't want to be a pirate.