Here you you can see the aluminum parts from histo:tec I use to prolong the intake manifold. This is a length of 40 mm. Parts are also available in 25 mm length. A longer intake manifold will produce more torque at a lower rpm range. This is to compensate toque loss below 3500 1/min due to the hot camshaft.
O-rings on both sides seal against the manifold and the Webers. The diameter is 40 mm (45 mm parts are available, too). Please check the necessary room in the engine compartment especially at Bertone GT cars with original air filter box. The carburetor support link and the throttle linkage must be prolonged, too.