I've got the ol cheapy Pro-M 75mm Bullet MAF and my 331 idles like a dream, even with a good sized cam and 24#ers. I do have a JMS chip, but it idled fine before the chip.
Just my experiences.
Are you sure you don't have a vacuum leak somewhere?
E
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1991 5.0 LX Coupe - 40,750 miles
331 cu. in. / Tremec 3550 / BFG Drag Radials
12.22 @ 114.31 mph - w/1.89 60'
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