Re: North Platte in April?
The river should be ice-free in the town by then and there are plenty of big fish to be had in Casper itself although I would stay upstream of the white water zone in the park. There was much contamination of the land that is now the park and golf course and although it has been cleaned up it doesn't seem to fish as well. I have had a 3lb cutbow just off Robertson Road so I know the river is full of fish almost anywhere.
As regards rod weight, it is all about the wind. I always have a 7 and a 5 with me. If it is windy I use the 7. Yes there are big fish in that river and you should not go too light with your leader because there are big fish and I have no reason to think the fish are spooky.