The tank would make a great SW setup (drilled or not). Personally I wish my tank was drilled... hindsight you know.

My only caution for going SW would be if you ever used Copper (Cu) medications when it was set up FW. Cu is toxic to animals lacking a backbone. It has been suggested that Cu can be absorbed into silicone and slowly leach back out causing problems. I'm not sure if this has ever been tested or proven but thought I'd put it out there. Personally I think a single Cu treatment wouldn't cause a problem but frequent or long-term Cu treatments may.

HTH,
Tom