A lot of it depends on the rear gears and tires. Don't be afraid to let the old gal spin. I would recommend keeping her under 2500 rpm for extended travel With stock gears and tires the truck will run over 90, just don't hold it there for too long.
Extended running at high rpm can cause rod stretch, usually due to the babbit in the rod ends. More damage is caused by excessive lugging at low rpm, when the engine knocks.
If you're rebuilding, definitely go with the insert bearing rods. I would also do a rear gear swap, your engine will last longer running at lower rpm.
