If it were me, I think I would not add any embellishments to the blocks ... the new would show as just that.
What I would do would be to square it up, and add sashings, as I wanted with coordinating fabric(s) or alternate with coordinating print blocks. When it came to quilting, that's where I would add any extra detail wanted on the butterfly ... such as quilting lines from the body out to the scalloped wing edge and perhaps across the body or a u-shape thru the length of the body.
Now you have me curious ... are all 8 the same fabric? Or do you have an assortment of colours/fabrics?
What a treasure you will have ... and fun to finish it off.
Now if only you knew the story behind the blocks, and who made them!