You are in luck. B&B Specialties has it and you can see it here:
http://bennettbuilt.com/page2.htm

Just to be sure, I have one of these stacks and I held it up to a Walbro SDC carb. It will fit fine. I fly a Brison 5.8 in my Ziroli Corsair without a velocity stack and do not have a mid range problem. It runs great all the way from idle to full throttle. I wonder if your carb atmospheric sensor diaphram is sensing some strange air pressure variances causing the problem. Also you may be able to help the problem with the carb adjustments. Maybe rcign will hop in here with some advice.

Ken