Suburban k20 motor

[wdbill]

I have a 1985 k20 suburban. It has a small block 350. I had put 2 motors into it since I've owned it both of the motors had catastrophic failure. It has a th400 transmission with 4.10 gears and 305/70/16 tires approximately 33x12.5. The mechanic told me not to go on the freeway. That the rpm's could have been what blew the motors. I put a tach in it and at 45mph the motor is turning 2000 rpms. He suggested that I put in a newer 4 speed transmission in it. He was telling me to put a 700r4 in it. I did some research and the 4L80E seems like a better transmission. And the transfer case would have the same 32 splines.

My question is what range of rpm's is safe. I know if I can keep It low that is better. Is 2500 rpms too high if I want to go on a 2 hour road trip? 50mph 2200+ rpms I feel like that is ok. Right now there is about 1000 miles on the motor. Was told to take it easy for 2000 miles and no pulling my trailer.