Get two pieces of scrap timber 100 x 75mm approx 1500mm long, get another two pieces of 100 x 75mm by 1000mm long. Make two T bars using the 1000mm section screwed to the top of the 1500mm section. When lifting the slabs with a buddy you can hold the slab in place with the T-bar which leaves a hand free to hold the screw-gun.