Very interesting, indeed! I have two identical MXL 990's and this got me really interested in modifying one of them with a 47 capsule. I'm not shy of soldering irons and this will be very budget-friendly.
Thanks for the post, @ljemusic
EDIT: But in reality, you're not building actual C12, U47 or U87's. You're building clones. But according to reviews, they're close enough.