What you stated is correct for a 65-66 Skylark GS with 401 engine.
With a 225/300/340 engine, the driveshaft is same length for ST300 auto, 3-speed manual, and 4-speed manual, and is same length as GS401 automatic driveshaft.
Now, does anyone know the correct length for a 65-66 Skylark GS with 401 and 3-speed manual trans?