Manifold sizing for Weber 40 carbs on a stock 2.7

[Porsche 2]

Preparing a rebuilt stock '77 2.7 Ltr 911 engine from CIS to Weber 40's carbs. Plan is to have WEBCAMSHAFTS regrind the cams to match carbs. Just received intake manifolds and discovered the upper diameters of the "pipes' are close to 40 mm which reduce down to 31 mm at the bottom. I am building it to be street/touring performance, not competition racing. What HP can I expect with no machined increase in the manifold and heads intake, and what HP can be gained with reasonable enlargement of the manifolds and matching head ports?