Have you tested the tank for leaks? Super important. I picked up a metal framed tank like yours and no matter how I resealed it it continued to leak... after a few weeks I gave in. Now it holds all of my aquarium supplies.
If it has the original hood its worth around $70 on ebay regardless of leaks.
Silicon sealant will never stick to the slate. Sometimes these old tanks can be fixed by pouring boiling water into them until the tar becomes soft and fills in the leaks.