do you mean the 4 lines that run from the little mounting block on the intake manifold to each cylinder ? I beleive this are the idle circuit, not really vacuum lines, so if the ID was less than original, the you would affect the idle circuit.
I think the only vacuum line (singular) on a spica engine is the big one from the intake manifold to the brake booster. I would think that this would be less sensetive to OE vs. aftermarket. Perhaps some later spica cars had vacuum advance on the distributor.
69 1750gtv / 73 2000spider / 91 Spider (daily driver)