You can drill a small hole (1/8") at the end of each crack, to relieve the tendency for the crack to travel further. Then keep it under observation.
After repair (by welding or replacement w/ upgraded crossmember), you can add these crossmember reinforcements, shown in green in this first photo:
Should still be available from Volvo
Vadis diagram:
These came standard on 960s, so maybe you can find some at a JY.