Both Hooker and Lemons told me 2 1/8" any thing smaller than 2" would restrict the heads. My heads are Bodix 10 RPSP with 2.150 valves and .734 lift cam useing 1.6 rockers about the same duration as your cam. I got very lucky and came across a set of Lemons 2" to 2 1/8" step headers with 4" collector which had an initial purchase price of $1350.00 with egt bungs. I got them for less than $500.00 unused, but again i was lucky and they just happened to have been made for a 1980 Malibu just like mine. my car weighs 3250lbs, but i will have to run a motor plate in the front and remove the stock motor mounts to clear the headers.
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