Full Throttle Al Al the math will not add up. 48 mm webers have a bore of 1.89", about 1 7/8". 58MM are 2.3". I have seen and been told that the 58 manifolds are nothing like the 48mm. You can use a 48 manifold and make a 58mm fit on it, but there are going to be a bolt pattern change and the mounting surfaces are too small without filling in the edges of manifold. I am not sure what Gene is maken now but 1/2" different in size is alot unless the base plates are being made small. I guess it depends on where you are measuring the carbs, top or bottom. Even the TWM's are not quite 58mm. I have measure the bores and plates, more like 56mm.
The tops are 58mm. Dave S is the only I know running 58mm, back in the late 60's there where only about 20-30 manifolds built for the 58's. This is the info I have found and recieved. These are also Cam-Am carbs. Gene may have changed the bodies since then. I have learned that size stated is not always the same size found out.
Rick L.