removing most of the divider was a common modification on LD340's and advised by Direct Connection back in the day. I think later LD340's came from the factory with the divider removed.

It helps performance by making the intake a little more hybrid, that is giving the dual plane manifold some single plane characteristics.

Nice modification work done on the iron intake pictured!

I've read you don't want to polish the intake ports, you want them a bit rough to help keep the fuel vapor in suspension/ atomized. Smoothing them out is fine.