If they are good batteries, and are free, I would certianly use them.
The 8D is a very stout battery, weighing in excess of 130 pounds with 168 lead plates. (according to an interstate battery website)
More lead = more heavy duty
They are not labeled as deep cycle batteries but i would be willing to guess that they will work just fine in that role.
With any batteries, not discharging the batteries past the halfway point is key to their long life.
Quis custodiet ipsos custodes (who will watch the watchmen?)