I haven't cast herring in a river but I use to have really good luck casting herring from the bank up in Washington at Point-no-Point. I found that cut-plugs don't last very long, and mainly used whole red-label size. I wdidn't use any weight and just counted down, then retrieved it very slowly. I would use a two or three hook setup, 5/0, 4/0, 3/0 is a nice hook combo with the smaller hook trailing behind the herring. I had good success with chartreuse or blue dyed herring, sometimes even dying the bellies red. Main thing to remeber is not to set the hook, just continue reeling and apply pressure after they take it.
Good luck