If that's to overcome restrictions on what cars first year drivers can use, then it would be safest to keep the added weight down low.
Keeping the tank full adds quite a bit of weight without any modification, then maybe some sheet lead under the carpets would be unobtrusive, and it's off the shelf at Bunnings. I'd be inclined to paint it both sides before dropping it in, and keep your tyres pumped well up.
I suppose the added weight on the suspension is no different than always taking four burly passengers with you.