Should I clinch or stake the under lugs after installing them on the barrel?
It is not strictly necessary since the fit is designed to be good, but you can further secure an under lug after installing it by using a screwdriver or flat chisel at the junction of the barrel and the under lug and tapping it down slightly, or by using a center punch to make dots on the outer corners of the under lug and barrel. Kibler's designs the under lug angles relative to the dovetail so that pressure sits at the top of the dovetail rather than in the bottom corners, making the fit more forgiving.