A few years ago Popular Hotrodding done a side by side test in the same car and build the 396 and 400 sb into 406 cubes. The big block was .5 sec faster in the 1/4 mile, and was more expensive to build. Plus if you have a small block in it now you will need to upgrade the coil springs,shocks in the front for the extra weight. Myself I like small block 400s any size.