Does the 500 has a sealed floor?
I used to have a sheltie and from what I read from Shetland owners site, they all recommended to replace the floor on their shelties (single skin, not sealed), Reason being:
1) it strengthen the structure of the boat,
2) Hull is not the thickest on the market and a small hole below the waterline will have you in trouble in no time (with a sealed floor it acts as a second hull, the boat will seat lower in water but you can still get home).
3) to add bouyancy to the boat, as the they can be prone to a sudden swell and swamp the boat.
Unfortunately their wasn't much info on the 500/600.
I hope you have better luck than I had with the Shetland regarding getting info for the 500, because the company has been taken over (serveral times), they don't really have much knowledge about the older boats.