Normaly you would get recomendation from the pipe maker, but BM is no more! I would start with 410's, needle in the middle position and keep reading the plugs. You may need to change your pilot, but I would not mess with it now. If it idles too fast you may need to drop a size. Twins pipes are hard to give recomendations for because they all have such different pressures and are not very forgiving. Twins need to be tuned a lot so you might as well get a good handle on how the carbs work. I have not run your set-up so do not take this as absolute fact, but I do think you will end up around the 400 mark. You may want to put one size larger jet in the mag side. If you have egt's it will help a lot, but plug reading is the most accurate. Good luck with it and maybe someone that has actually ran that set-up will chime in. Black Majic made mostly flatland pipes.