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I think the original carb was a 4428S for a manual trans or a 4429S for an automatic. It was an AVS style carb made by Carter and is rated around 750cfm for the 440. There is also a possibility that a '68 440 motor came with a 3918 Holley with vacuum secondaries rated at 650 cfm, but I wouldn't expect those to come on a GTX. The 383 and 340 used a smaller AVS carb rated at around 625 cfm.
the 340's and 383's used 625 cfm.the 440's used 750 cfm.i have a 4429s 750 cfm,but it is corroded bad inside and wont run worth a dam.now its just a paper weight.