Despite the remark, there is no "universal carburetor", in truth what they sell is a compromise of various sizes, circuits and internal components which must be tailored (by you) to suit their individual engine, gear, stall speed, driving style etc. The 216, 235 and 261 need slightly different innards just because of the displacement, as well as port sizes, cam timing, etc.
As said, the mixture distribution is best with either the original manifold, or 3 (almost separate) manifolds, since 2 X 1 frequently favors the center port (#3 & 4 cylinders). This cannot be adjusted out, but requires internal modifications to the manifold internally (dams, popsicle sticks, drain grooves etc.).
It's not a bolt-on, it's an engineering project, and many people are sick of it before obtaining acceptable results.
What about a 2-1 adapter and a 2 bbl.? Yes, the extra air (not gas) will increase power, but the original manifold's distribution doesn't transfer over to a new base very well, and the adapter is very tall.
If this were my engine and I wanted a project, I would try making a manifold similar to the Corvette to mount 3 X 1-1/4" H2 or 1-1/2" H4 SUs.
