There are no downsides to a neutral balanced engine.It is not really needed for an everday drivers that will never be run hard. But if you plan on turning it up past 6000 RPM you might want to do it. I ran alot of Chevy 400 engines back in my circle track days. We always turned the engines around 7200-7500 RPM and the ones we had internally balanced always seemed to show less bearing wear upon teardown. They just were smoother running engines.
69 Mach 1 427 Windsor,T-56 6 Speed,4.56 Traction Loc