DT466E - issue or just built weak?

Hey everyone. New to the forum. We just bought about a month ago a 2000 International 4700 service truck with the DT466E and 6 plus 1 tranny. It is a tire service truck. Cluster says 36K miles. Don't know if that is correct or not. To my questions. I have searched and searched and everything related post I find online is either a bigger problem or more problems than what I've got. It seemed a little sluggish and every once in a while it would cut out for a split second and then kick back up. I just serviced it yesterday finally and changed the fuel filter as well and also cleaned out the little strainer filter and plastic bowl by the filter. It was definitely needed. Runs like a whole new truck now. Hasn't cut out at all and drives much smoother. But I feel like I am lacking power. Not necessarily through the first 4 gears but mainly 5 and especially 6th. I'm in 42-4 mph when I shift into 6th and it takes a whole to get to 65ish and is also very weak going up a couple of the grades we have here. Is this normal for this truck or is there a possibility of something else going on that's restricting it? If not, are there ways I can increase power slightly without causing any problems.

My 2nd question was I've read that there is supposedly a 2nd fuel filter on the drivers side frame rail. I cannot find any other filter on it besides oil and coolant filter. Is it correct that I only see 1 fuel filter?
So overall, I feel like she idles well. Starts well - cold and hot. And I feel like she drives pretty smooth for the most part minus the feeling if weakness in the higher gears. Oh and also replaced the air filter when I serviced it. Thanks in advance!!! I am new to the DT466E