Explain "Neutral balanced"

[Candoo]

Other than the literal translation what is the process to "Neutral balance" a 351c and any downside with regards to a stroked 408.

[Burn-Bros]

A machine shop will insert heavy metal slugs into the counterweights to yield a "zero" balance. You would then require a neutral flywheel and harmonic balancer as opposed to the 28 oz. flywheel (not sure what amount of imbalance the cleveland has).
There is no downside to it. Some cranks may not be able to get to zero depending on counterweight sizes and the weight of the rods, pistons,etc.

[C0V3R]

Is a neutral internal balance better than externally balanced?

[Burn-Bros]

It is more advantageous because you can replace flywheels and dampners without getting the rotating assembly balanced again.
some engine builders believe that externally ballanced engines exhibit more torsional flex because the balance correction is at the extreme ends of the crank.

[mod9]

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.