Paxton option in 66? Yeah...right.....

40 horsepower sounds about right - about what basically the same supercharger gave the R2 Studebaker 289 in 1962-64. Not a whole lot of boost - maybe 5 lbs or so. The R3 Studebaker had another 50 horsepower on top of that; it had a hotter cam and better heads, but also a supercharger with a lot more boost, which is why its carburetor was enclosed in a pressure box. Below: R2 on the left, R3 on the right.

R2 Studebaker.jpg R3 Studebaker.jpg