Bill, Our stone labyrinth is not quite on the scale of those monoliths at Steness but is a nice piece of work. If you are thinking of doing a smaller scale of "standing stones", the tractor will definitely come in handy.
We have placed a few (smaller) standing stones around the property and you may find, even with a substantial tractor, that you are unable to actually dig the neccesary holes AND stand the stones up. We did it tandem style -- backhoe digs the hole, tractor moves the stone into place, tilts it up and "holds it" while backhoe backfills around it!
Make sure you have solid stone that you really trust the integrity of and that you really get it firmly "imbedded" into the ground, our biggest ones (4 -5ft out of ground) are plenty heavy and would be more than plenty nasty if they toppled!
I'm afraid that one of these coming down out of control would wipe out even the HD brush guard - not mention the rst of the tractor.....
Our biggest stones are probably 1/3 the size of those pictured at Stenness and I can barely move them or grunt them up with the tractor (TC35) -- chains, bucket or dragging!
Good luck!!
Steve