The long runner, small plenum design does seem to be the right direction for a performance Y intake. As long as all the port openings are at the same depth below the carb butterflies, fuel distribution should be reasonably good. Here’s a picture of the manifold I’m currently running on the roadster which is very similar to one that's pictured at least in its outwards appearance.

The smaller plenum will have a higher velocity and improved fuel distribution. The lowend torque numbers increase substantially with the smaller sized plenums. In the right dual quad application, a larger plenum could be made to work assuming the port layout is such that the ports under the carb are inline with the length of the engine rather than stacked with the head layout. Here are some pictures of large plenum intakes that have been both raced and dyno tested. These have the stacked port layout under the carb which simply hurts low end performance. The short runners are also an issue in that reversion within the plenum when using reduced lobe center camshafts comes to the forefront and especially on cylinders 1 & 2.