Ok, I watched your video (thanks for sharing with everyone) and now I believe I can tell BB K-Frames. eBay one above was a Direct Connection Big Block as you stated. Sold now so I am out of luck.
See attached photo. Red Circle area on a DC K-Frame will be closed up by metal plate and the red arrow area of the spool mount support will have a straight triangle side. A home made V8 to BB K-Frame wont have a straight triangle side where the red arrow is and will be set back where you cut the mount to move it back.
Also, I was concerned about what you said in your video about the strut rod holes being oval and the shimmy at high speeds. Was that pertaining to that particular K-Frame or inherit to DC BB K-Frames? Wasn't Sure.