Hi. I forgot to answer you other question, about maximum speed in 3rd if you don't get your OD fixed.

The sailboat that I usually tow with my '84 Volvo (my wife and daughter have '93's), sometimes several times a week, during the summer weighs about 1,000 lbs, and I lock out the OD about half the time while on the highway (i.e, on the upside of the hills).

Also, I occasionally have towed a ~3,000+ lb powerboat with my Volvo on the Garden State Parkway (relatively flat), mostly in OD locked out, and at 55+ mph (or else you're just driven off the road by the other GSP drivers going 70mph).
Finally, the heaviest thing that I've towed weighed about 3,500+ lbs (a big, tandem axle U-Hall densely packed) up and down the hills of CT and MA, between NJ and Cambridge, MA (i.e., Rt. 287, to 684, ... finally to Rt 90 Mass Turnpike). Anyway, to make the grades on this roads, especially with a heavy load and hilly roads, I've never hesitated to keep it locked out of OD most of the time -- that U-Haul trip to Cambridge (to take my daughter to college) was almost 100% in 3rd gear!

All of this, on highways, was maintained at 55-60 mph to safely keep up with traffic (except when the car couldn't make it up some really steep hills). I have no doubt whatsoever that 55-60 in 3rd is fine -- although I admit I haven't done this for 1,400 miles.

One thing to consider -- whereas I usually get over 20 mpg, when towing that U-Haul almost continually in 3rd, I got only about 8 mpg (a lot of stops at gas stations)! So 1,400 miles will be a LOT of fuel, although you won't be pulling a heavy trailer too, so it probably won't be as bad as that.

Good luck. But remember to be suspicious of that work that they claim your 1989 car needs! I'm not buying it.