1988 Okanagan Econoline 250 Question

Hi all - my Grandmother and I just returned home from picking up a 1988 Okanagan Econoline 250, with 96k original miles and extensive service records. We paid a good price for it, as it needs some cosmetic work, but it has good bones. With that said, I had a quick question about the 5.8L 351w and 3-speed transmission combo. How easily should these rigs drive through the hills/mountain passes? On our way home, going over a couple hills, the 5.8L really couldn't maintain 55mph. It seemed happiest dropped down in second gear doing 45mph. All gauges checked out, and the engine didn't sound like it was struggling, though there was a high pitched squeal that kicked on a couple times, that sounded like the belts at higher RPM. Is this normal? I know that the conversion kit adds quite a bit of weight and I wouldn't be surprised that chugging along at 45mph over mountain passes is normal, I just wanted to get some insight, considering I'm a newbie.
If everything checks out to be fine and dandy, are there any recommendations on gaining a couple more horses and low-end torque? I'm pretty mechanically inclined, and any thoughts, questions or concerns would be much appreciated!
Thanks