Some will put them together without the end caps. Early production Moog Offset bushings didn't include new end caps. Now for me... I can imagine the end of the bushings center tube digging into the chassis clips and possibly working loose over time. Just my imagination though, never seen it.
I would rather have the OEM type end caps with the little bite bumps on them.
Bottom line, you have 2 options,,, Put it together without end caps or use a shop hammer to spread the chassis clips until the arm drops in with end caps in place.