On the older Rams,with the 4850 lb axle,most plow mfg's would only recommend a 7 1\2 foot poly plow. Newer Rams are rated for an 8 ft with the 5200 lb axle. You will need a 9 ft for the 3500. You will have no problems with a 9 ft plow. I would recommend a western,as they are one of the lightest,and pretty strong. All the Boss plows are very heavy,especially the "V" plows.
You should have no problems with the springs,just add a set of Timbren load boosters. Most of our plow trucks are older,with the 039 springs,and are fine. Your truck will have the heavier 047\048 springs rated even higher.
Adding ballast to the back of the truck,will pull wieght off the front,to keep you under the front GAWR. I have a steel\concrete rear bumper that adds ballast and offers extra protection. With a 9 ft western,upgraded batteries,full tank,and all my other Gizmos's,my front axle shows 4820 on the scales. Only problem I have is I'm now over my GVWR. You will be fine as the 3500 has a higher GVWR.