I think your idea of sewing them onto blocks and adding sashing is a good one. Because they are 13" in diameter, you might want an even larger block. 13" on top of a 16" block will give you a pretty narrow frame around each one. I would take try to find some scraps and lay a couple out to see how it looks.
As to what fabric to use for the blocks. You certainly can go with a solid, but you might want to look for a subtle small all over print that "reads" like a solid. I'm not real familiar with '30's fabrics so I don't know if they have small subtle prints.