[Darrell Ratchner]

MrMike, I don't have access to my Holley book at this moment but I think the 550's are really designed for drag racing and maybe even be a center squirter that opens all the barrels at one time. If this is so then you definately do not want it for your cobra, unless all you do is drag racing. Also you need to check because on most of the front to rear 2x4 intakes you need a carb without a secondary metering BLOCK. The carbs with a METERING BLOCK are too long. ie they don't fit the manifold.
The 450's have a METERING PLATE, NOT BLOCK, ie thinner, that fits inside the float bowl.
The 450's # 0-9776 have a metering plate. The other thing you need to understand is that you don't need anything bigger than the 450's. 900 CFM on a 428 is gobs for the street. The smaller barrels on the 450's increase velocity and driveability.
Gotta go now, I'll look up the 550's asap.

Darrell