Get the 1 5/8", they'll be better matched to your exhaust ports. If you had a 351, or a 347 stroker the 1 3/4 might be the way to go.
I'd expect you to pick up 3-4 tenths with those mods. The headers will probably give you 10hp, the prochamber H 20hp, and the rest of the exhaust will chime in a little too. More than that, you'll gain low end with the headers.
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