Leaf springs are pretty easy to deal with. You can make up brackets and pretty much mount them anywhere you want them. You just need the center bolt to end up where you want the axle centered in the wheel opening, and be sure the springs are centered side to side and the correct width for your spring perches on the axles. I suppose being square to the body center line would probably be good as well.
Personally, I would go for an over the counter spring set as close to the length of the springs on the rear frame you are using, and support the weight needed. According to the Standard Catalog of Chrysler, the 56 Dodges weighed in between 3,300 and 3,600 lbs, 1,700-1,800 lbs springs shouldn't be much of an issue for a good spring shop.
When I did my 54 pickup, I bolted 1/4" thick 4" x 4' angle to the side of the frame and bolted the spring brackets to the angles. Didn't give me any problems for the 50K miles I drove the truck. Gene