A while back I had a hydroboost rebuilt and it turns out the hydroboost rebuilders do not include the pushrod, spring, and spring retainer that goes between the hydroboost and the master cylinder. If you made the mistake of sending yours to be worked on, like me and didn't take them out, or if you purchased your hydroboost separately, you need those.

They are incredibly difficult to find. I noticed that other forums and boards had posts where no one could find them.

I'm posting this as a pay it forward. If you are missing those pieces and searching everywhere hopefully you find my little message in a bottle.

This group has them:
https://powerbrakes.com/

Good luck in your build!