I think you're off to a good start, actually! I purchased some vintage blocks from a reputable seller on eBay, and also bought the white and other solid colored cottons - they're highly likely to be the same age and likely the same quality, which increases the chances of the fabrics' wearing the same way. If the weave density of your muslin and the blocks is the same or about the same, it should be okay. I'd use 100% cotton batting with no loft. If you have feedsack blocks, I'd pair them with a like-weave feedsack sashing fabric, rather than a muslin, which has a tighter weave. As an extra precaution I'd wash the blocks and the muslin in Orvis soap, block while wet and air dry - IF you haven't done this already.
Your projects sound wonderful - have fun for all of us!