I'd take a dremel tool or die grinder to the one MAP sells for the Atlas and do a port match to the carburetor throat.
Anecdotal BS:
I made a twin carb setup for Mikuni 34mm carburetors out of what was available for the Commando back in the early 90's. I couldn't find anything for the Atlas at the time. I hand filed the flanges at the carb mount end down until they were round. Then used a small round file to make a little indented ring all the way around, like the indented ring on the Mikuni intake end. I used automobile fuel filler hose for the rest of the manifold. The carburetors are horizontal. It's a bit tough on the throttle cables, but it does work very well. No hiccups and peppy with 3-inch velocity stacks on the carburetors. The fuel filler hose makes it easy to change the length of the intake. My filler hose sections are at the minimal length. If they were any shorter, the mounting flange on the fuel bowls of the carburetors would touch, and probably rattle a bit from vibration.