Congratulations on a great score! I've just recently started shooting slugs again after getting a very good deal on a Browning A-Bolt 12ga.
The slug I settled on was a Remington Premier Accutip. It's a sabot that slings a 58 cal tipped, bonded 385gr bullet at a claimed 1900FPS, providing over 3,000 FPE at the muzzle.
It is very accurate @ 100 yards out of my Browning. I caught them on sale at Midway and also got free shipping.