Well, I don't know how steep the mountains are there..... but I'd worry about the tranny. Depending on which one you have, Sables were not known for their trannies (assuming you have the 3.0L). I've heard if you have the 3.8L it was the better tranny.
Depending on the grade, I'd take it slow. Keep it out of OD if you can on the hills (so it won't hunt). If flat, might be okay. Watch you temp guage. If it were I, I prob would not go faster then 65 or 60 on the hills. Flats, I'd prob. push 70. Down hill, remember you have no brakes on the trailer. If you can, go 60 and if possible, take it out of OD so there is a little engine compression to help slow.
If your car is anything like my old car, then the tranny cooler is a joke.... or was it the power steering cooler I saw?
Any now, if I recall, it's just a long tube that winds back and forth in front of the rad. (aside from the normal radiator cooling). If you want, you could add a tranny cooler to it. It should be under $35.00 or so I would think. Fairly simple, especially if the "factory" cooler is in front (should be just a matter of a hose clamp). If you have an actual cooler, even better.
Depending on when you go (ie outside temps), I'd take it slow. But that is me. Last thing you need is a busted tranny and on the side of the road.
Looking at the online stuff, 1250lbs seems to be the rating. I'm sure that the trailer is much lighter then the bike. But, don't forget to take into account any extra stuff you load INTO the car (ie passengers and luggage).
So your trip will actually be closer to 1200 miles total (round trip). It should be fine, but just keep an eye things. Bring some extra tranny fluid and check it everytime you fuel up (for color and smell). Before you leave, you might want to have the fluid changed in the tranny.....