That motor will run all day a 3500RPMS, If you....

1. don't let it detonate. = fill it with 93
2. don't let the engine get too hot
3. don't let the trans get too hot

With that said, why not just back it down to 40-45? I personally don't think there is any shame in not blazing up a hill at 70mph. Hell, I drive a semi, people blow by me all the time going up hills.

 

These engines are desinged to run at higher RPM while under load. As long as you keep up on changing your fluids and I would say to run full synthetic you will be good. The concern that you should have is the tranny and running it at high temps for that long. The best thing you can do is install a good trans temp gauge and stay on top of changing your tranny fluid. If you make that rip often I would also look into running a larger trans cooler and possibly a deeper trans pan to increase the amount of fluid which will help to reduce overall trans temp.

 

It's really your call, it's next to impossible for any of us to give an accurate "do it" or "don't do it" over the web. Like others said, just watch your temp, and if things start getting a little hot pull over and let it idle a bit. Turn off the A/C and put down the window for a few minutes. Running premium wouldn't be a bad idea either, or add a little octane booster to the tank before you start towing.

I think the best thing you could do is just back down on the speed a little. When I tow a double sled trailer, probably about 1500lbs with my 300 and 5spd, I just put it in 3rd and get content with being in the right lane going up the hill at 45mph. What's the big rush to get up the hill? Back down to 40 or 35mph, you'll still be at the top in well under half an hour and you can make up the time doing 100 back down it. Or tell the wife to lose some of the crap and lighten your load