The frame is threaded for the THREE bolts that mount the bracket to it. I can't remember the size. I'd remove the loose one and take it to the hardware or parts store to get a duplicate (one of of the correct grade as well as the correct size). If the Volvo were my only transport, I'd drive it to the parts store, remove the loose bolt and get a duplicate, then install them both before departing.

If the bolts you mention are the two that mount the A-arm to the bracket, they are secured with nuts. Small fingers required for the rear one, or you can temporarily remove the rear bracket, mount it to the A-arm with bolt and nut through its bushing, then remount its body on the frame with the three bolts.

The Bentley manual specifies the torques for all the front suspension bolts, and cautions that the A-arm bolts should be torqued with the car's weight on its wheels, lest the front bushings be clamped in the wrong position, setting them up for early failure.

Maybe someone else has the size and torques at hand.

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jds