You need at least 1 5/8" primaries on your headers with 3" collectors. After that, I'd say 3" would be the way to go. If you want to go smaller, I'd say 2 1/2" is the limit. So either one of those, 2 1/2" or 3". Should work great.
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Previously "BennyBoy"
'69 Coupe with 351W-I rolled it 8/8
Edelbrock RPM intake, Edelbrock RPM cam, Edelbrock 750 cfm carb, Hedman headers, 2 1/2" pipe, glasspacks, C4: shift kit, 9" Currie: Trac-Lok 3.70s, 290hp-stock heads