I would go to ND. And yes, the spring would be the best time.
The first thing you need to do Is make a scouting trip out to the location you pick. And it might be too late to do that now. And you should get on the internet and look for plat maps for the area you plan on trapping. This will give you all the landowners names and locations. Also, will show you all roads and water sources. Don't go long lining without these maps.

You can trap state and federal road right a ways In ND and some county roads. Once you pick a spot get a hold of the Local DOT office and ask questions.

The thing I found in the area i trapped most of the rats lived in bank dens. Finding those dens was a waste of time. You just need to gang set those ditch banks and set feed beds and toilets. And if your area has lots of culverts colony traps are the answer. I made 8" traps and then made 8X8 wings to fit on one end. That way instead of putting more than one trap in the culvert you only needed one. The best culverts were the ones that were completely filled. I think in the partially filled culverts the rats swam on the surface and not on the bottom.
In my opinion floats are way too cumbersome and they take up way too much space in the truck. And they don't work in some areas.

We used rat stools for about 80% of our trapping. These were platforms on stakes. and they were adjustable for different water depths. Your traps by law have to be at least 2" under water. I learned the hard way.

My advice would be to wait till next fall go out and do some fall trapping and scouting then go out the next spring to do your serious trapping.

I stopped going because you're going to lose money. With low rat prices and finding a place to stay would be a money loser. I even had a great place to stay and it was only costing me $300.00 a week. But rats were bringing $7.00 averages. And we were checking twice a day.
Some days we were bring home 300 rats a day.

It will be a challenge but what the he33 go for It.