Sorry, I didn't read your post thoroughly. You said, ECE drop spindles, and I was assuming a caliper bracket. Your knuckles have spindles and a forged caliper bracket. I had to check out this post for photos:
https://67-72chevytrucks.com/vboard/...d.php?t=806491
I understand the inside of the bolt holes is a rough surface, but there are two machined surfaces that encompass the caliper that are machined. You could check those for parallel to the rotor. Seems like something changed from the initial installation of those knuckles. I still think some heat and pressure could solve it. Good luck.