Thanks for that info, it confirms my CAD specs for the port dia. So I take it then the 58 mm IDA's just have less to seal the carb to the intake? If the bolt pattern stays the same, then that would mean there is less support on the bottom. But they are 15 mm wider between the center of the carbs? The 48 mm IDA has a spacing of 90 mm.
This is the diagram I have found to help with this project. The pic is for an IDF, which is identical to the IDA in footprint as far as I can tell. This diagram is for a 40 mm bore. What I've read is that the footprint is the same for the 48 mm bore.
Since the needed IDA is a 58 mm, that means the air fuel mixture would speed up and decrease in pressure due to a 15% increase in area. You want higher pressure, not lower pressure, in an intake. Creating an expansion chamber right before the port might be a good idea, to still create a high pressure area. A holding tank of sorts for the mixture. This is just an idea though and I really don't know if it would be better than just having a gradually decreasing area through the whole intake runner. In theory, an expansion chamber will help provide the engine with incoming air and fuel. Which brings me to the idea of using a nozzle shape at the outlet port of the intake rather than easing the shape narrower. This causes the air fuel mixture to have to funnel into the intake ports on the head, like having a reservoir. That is one of the reasons why most 4 bbl carb intakes have a valley instead of individual ports. This allows you to have a higher pressure region inside the intake. Lots of different ways you can design an intake to hold the same carb.
Of course I could be entirely wrong in all this thinking. From what I know, I'd think that a nozzle fed by an expansion chamber would be beneficial to the engine. The expansion chamber would go from 58 mm to say 62 or 64 mm in a cone shape. Shortly after reaching 62 or 64 mm, the nozzle would reduce that to 54 mm, a decrease in 8 or 10 mm. This would create a high pressure region right before entering the intake ports in the heads.