Just adding my 2 cents .... I would use a solid sunny yellow for sashing. Using a solid color would be true to the time period that the blocks were made in. I think adding more retro fabrics would be too many retro fabrics. Please let us know what you decide. :)

I had some like that on really thin muslin. used the lightest weight interfacing you can buy (and it is light!)
gives them some stability & you hardly know it is there.
30's prints would be great. you don't have to stick to solids. could do log-cabin style blocks around each flutterby. and use lots of different prints. will be so pretty when finished !

[I agree with using a pastel solid for the sashing, in keeping with the period.

now that repro vintage fabric is available I would look into using that. I would sash those butterflies (2") in one of the "olde" pastel solids and use a reproduction vintage in the cornerstones of the sashing, then another repro fabric in the final border and bind in the same solid as the sashing...Just what I would do....

The brown that you were contemplating or black would really give the quilt the zing. If you don't want to sash what about alternating pastel block, maybe aqua. If you do a border a 30's print that would compliment

Update! First, let me say that I really appreciate all the suggestions and input from you ladies. After weeks…ok, months….. of thought and planning, I searched and found some great 30s reproduction fabric with EVERY color in it to correspond with the butterflies. (BTW, I only photographed 4 blocks for the initial post when actually there were 30 blocks (blue, green, pink, yellow, brown, orange, and red).) My plan was to sash with black since the butterflies had black bodies and the wings were embroidered in black. When I got close to cutting the border, I felt the 30s print was just too distracting…. I didn’t want to go with a solid (too boring). I had a one yard cut of Moda Wildflowers II and it just looked right. I know….it’s not keeping with the period; but my friend is not a “purist” ….and after 78 years of the blocks being in storage, my goal was to get the project finished, so she can enjoy it. So, I went to my LQS and Lo & Behold, they had the Wildflowers fabric! I bought 2 yards to keep the ‘landscape’ directional. I used a 200 psi threadcount sheet in a muted, mint green color as backing. I finished the project tonight at 6:30PM, adding a label to identify all parties; and delivered the quilt to my friend at 7:30PM. She was so happy, and we were both bawling in a matter of minutes. This was my Christmas Blessing! Thanks again for your input. Merry Christmas!!!

You did a beautiful job.

That is just gorgeous! What a perfect choice for the border, and I can see that both you and your friend are thrilled with the results. A wonderful thing you have done....