If you have the money you can have the 3.7 engine block made of billet aluminum with full water jackets and nicasil liners, get a billet steel stroker crank made, titanium connecting rods and forged pistons, have billet cams made, titanium valve-train components and ported heads, a hogan sheet metal intake manifold and a pair of turbos that would probably blow 1600hp through the thing. The list of things required would be enormous, but you could end up with an engine that displaces over 4 liters and spins happily to 8500 rpm.
Of course you'd be talking $9,000 for just the engine block. Probably that much again for the rotating assembly. More than that for the turbo set up. Another $3500+ for the manifold. So on and so forth.
And it still sounds like a V6 and is missing the bottom end torque of the V8... It's simply more economical to start with 2 more cylinders...