Want to insert Klokov press into 5/3/1 for Bodybuilding - where should I put it?

Do it as an assistance exercise on either Bench or OHP day.

If you want to do it on the day you've outlined, you just need to find your balance between what's practical in terms of time in the gym and fatigue. Standing OHP, Klokov, and DB OHP are all similar movements, so you don't really need all three. You could rotate lifts every couple 531 cycles, e.g., do 2 cycles with Klokov and then 2 cycles with DB OHP. Or you could put one of the lifts on Bench day. Or if it's not having any negative impact on time / fatigue you could do all three.

Personally, I wouldn't replace Standing OHP because that's your primary lift. If I was keeping in lateral raises, then I'd choose one or the other between DB OHP and Klokov.

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