The crispy feel could be starch. Or even fusible. Even though the fabric is 1930 feed sack that doesn't necessarily guarantee the blocks were made then. They could be made later and the maker used a fusible interfacing to adhere the butterflies before blanket stitching them down. Hard to tell from a picture. Either way, as long as the fabric isn't falling apart with handling you shouldn't need to stabilize them further. Once in a top and quilted adequately they should be fine. But if you are truly concerned you can reinforce each block by sewing it to another square of muslin or fusing. I would opt for the former as some fusibles tend to be stiff.
It looks like you have more than enough blocks to make a decent size top so you may be able to eliminate the ones where the applique is too close to the edge (less than 1/4" where part of the applique will be buried in a seam allowance). To make a top I would measure every block and at that time establish which blocks where the appliques are coming dangerously close to the edge. I would then square up and trim every block to be the same size based on the smallest block. At this point you can determine how large a top you want and if you want or need sashing strips. I would use muslin for that and for setting triangles if you decide to set on point.
Have fun!