With a true dual system, 2.25 - 2.5 inch would be fine as long as you don't choke down at the mufflers. On my 2 ton with the 283, I used 2 in. pipe with 2.25 in. mufflers, the manifolds aren't 2.5 in. so it's not restricting at that point.
1957 Chevrolet 5700 LCF 283 SM420 2 speed rear, 1955 IH 300U T/A, 1978 Corvette 350 auto, 1978 Yamaha DT175, 1999 Harley Davidson Softail Fat Boy
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