Without knowing the condition of the vehicle and the motor, all we can do is guess - if you have a concern that the vehicle is not up to it, then don't take the chance.
Running a well maintained engine, in good condition, at 3500 rpm for long periods will not cause a problem - on the other hand, if it's worn and burns oil, you'll need to keep stopping to top it up, if the cooling system is not up to scratch, you do run the risk of it overheating - I'm sure you get the picture - it's all about what condition it's in - age & high mileage suggest a worn engine, but, you've given us no real details.
I occasionally take my 1992 Sidekick with 150,000 miles (240,000 km) to visit family in northern Colorado (and some 4WD in the mountains), about a 1000 km. drive. 1.6L with 5 speed manual transmission, 2-door and no A/C. I have experienced no problems. Runs very consistent, about 24-25 mpg gas, temp always less than 1/2 on the gauge (closer to 1/3 scale), no real loss of engine oil or coolant. While internet speeds are 75mph (120 kph) or 4,000 rpm, I normally maintain around 70 mph for around 3500 rpm where it just seems to be happier - its "sweet spot." I keep mine well maintained, has good engine compression, and always gets a 4-wheel alignment with new tires. Of course, it will be noisy, especially with a rag top, but so it is at 30 mph. I do recommend changing the oil and check engine coolant, proper tire pressures, etc. before the trip. Good luck on your trip and hope it's enjoyable.