Are they slides in the Mikuni carbs or throttle plates, vacuum dampened, or directly lifted? The OEM had throttle blades and the slides were opened by vacuum by a diaphragm. This kept the throttle opening smooth and made the take off less jack rabbit like. I had a cycle innovations conversion, an my 1100. It had an aluminum plenum that dropped into the space between those rubber sleeves in you image. Just had a single barrel Holley carb, much like one that would have been found on an early 60s Ford 170ci 6 cylinder. Cycle Innovations had them specially made with a spool for the throttle cable, and a 3 layered primary booster venturi, to give it a smooth transition from idle to high speed circuits. I gained 1mpg and lots of bottom end torque, would really scoot when you got after it. Only drawback was carb/intake icing on cool damp days, but once the cooling system got warm, it went away because all the heat from radiator was dumping on the intake and carb. The single carb was awesome, set it once, never adjusted again. I had it on there for 14 years, with no problems.
Finding an acceptable carb for a DYI conversion could be tough, gotta look for something pre-emision, maybe solex, Early Datsun pickup, something around 1800cc engine.
Maybe even a progressive 2bbl. with vacuum secondary. The intake plenum could be fairly easily made. Just a box with 4 runners, single plane.
I never cared for the 4 carb setup, I would OH the carbs, set it up with manometers, and next time I road it, everything was Fubar.