Bilsteins 5100 on 75 k10. Rear shock mounting fit issues

The Bilstein 5150 rear shocks (24-187084) for my 1975 lifted k10 (4 inch) have an upper mounting bolt (M16-20) that is larger than the stock (1/2-13). I want to verify this before removing the old shocks. I can't find any info on this as most people who have these don't report this fit issue. I am hoping this is an upgrade from stock and, once removed, I'll see that they fit OK. Appreciate any insight here.

That’s the same part number I used on the rear of my ‘80 K10, I don’t remember any issues. Be sure to check for cracks in the frame while you’re in there, mine was cracked on both sides where there is a little dent and flat spot pressed into the frame.

IIRC @Grit dog had issues with how the bilsteins fit but I don’t remember what the issue was

ETA: I put a set of cheap aftermarket shocks on in the late ‘90s, I guess it’s possible I drilled out the mounting holes then but don’t remember

As I recall, which is getting harder as the years go by, the frame holes for the rear shock studs are 5/8". So ya a 16MM would be the right size.

Best bet, return the Bilsteins and buy shocks that fit.
I’m sure all the issues I had are posted here somewhere. But suffice it to say, ordered for 4” lift. Front and back were way too short and both had fit issues with the bolt sizes that would have required pressing out the bushings and finding new ones and the steering stabilizer didn’t work either without modification.
Honestly unless you’re going for 100k miles on your next set of squarebody shocks, just buy some (for less $) that fit.
Bottom line, they don’t make shocks specific to antique trucks and the “recommended” ones are just the closest to working with modification.
And what I found for required length was basic 5100s aren’t offered long enough even if I wanted to beat them up changing bushings etc. Had to move up to the 5125s or whatever. This was a K20 but suspect similar for K10s. ORD even offers different bushings to make them work on squares.
Skyjacker on the other hand has a very comprehensive application chart that is easy to follow and they will fit without issue.
Hindsight, glad I returned $600 worth of wrong and got $300 worth of right that actually look better and work just fine.