Bolt on to what? Chevy's don't use bolt on pickups. They're a press fit into the bottom of the pump...a shitty press fit at that, and some of the pickups have that little novelty screw hole to make you feel better. All the little screw hole does is prevent the pickup from falling off of the pump.
The issue is that the press fit aftermarket pickups don't seem to seal for shit in the pump. If there is even the smallest air leak, the pump will loose prime and oil pressure will drop when you start winding the thing up. Weld all the way around the pickup, not for structure but to seal it and eliminate the possibility of air being drawn through where the pickup meets the pump. Then try it again.