I flew IAD-BRU last saturday. Flight arrived about 35 minutes early. I had been monitoring this flights the whole week before and noticed it was nearly always early.
Flight normally arrives at a gate very close to passport control.

You will have to change terminals, the walk is long, and you will have to pass a security check again. There is a fast lane, but not for premiers of all airlines.

1 h 40 minutes is more than enough time to transfer.

Luc