I just purchased a 1997 G3500 with the 6.5LTD.

What should I do immediately to improve reliability and performance?

I already removed the airbox silencer.

We plan to replace the soft drink straw of an exhaust system for a better one.

what would a 6.5 4 inch straight piped van sound like?

Probably similar to a 6.5 Truck with 4" exhaust.

However, for posterity, I will post a video for you guys.