It's not a downgrade. A 258 operating at 70% volumetric efficiency, which is generous for an emissions laden 258, would require about 280cfm at 4,500rpm. The big 2150 is about 290cfm. I use the 4,500rpm in the figure because that's as far as the factory cam will pull and the crappy intake port won't go much further. If you are winding further or have manipulated valve-timing and flow ability past that point, then yes a 400cfm(or whatever) webber would obviously be better. On a completely factory engine, the 2150 is where it's at. Once you need larger.....skip the toys and get a 390-600cfm holley.