Except you were not in Christ before the foundation of the worldWould be in Christ? The choice is made "in Christ". The text is past and present tense. There is no "would be".
Ephesians 2:11–12 (ESV) — 11 Therefore remember that at one time you Gentiles in the flesh, called “the uncircumcision” by what is called the circumcision, which is made in the flesh by hands— 12 remember that you were at that time separated from Christ, alienated from the commonwealth of Israel and strangers to the covenants of promise, having no hope and without God in the world.
The idea is those who become in Christ would be chosen to holy and blameless
That is what God determined before the foundation of the world
not that unconditionally selected men would be put in Christ