N14 redtop low power

Got a question on an N14 red top 98 year model motor rated at 525. Odometer is reading right around the 837k mark 13 SPD with 3.90s out back loads are around 90-95k my question is seems like this tractor just refuses to pull period the end. I'm not a Cummins person don't know anything about a Cummins really, but I would think a 525 would pull a lot better than it does. I have learned to run this motor up to about 1800 and not lug it below about 1400 on a pull as it seems to just dog out but even with the 13 over and 3.90s seems like I'm dropping more gears than I should have to on the hills. Im currently trying to figure out if its just a tired motor or I'm not doing what I need to or am out of the operating range on hills. I've noticed a lot of blow by from the tube, and on the passenger side on the brake cans there's spots of oil which I'm assuming is coming from the tube which would indicate a tired motor that's about due for a tare down.

Am I doing something wrong with the operating range on this truck or is the motor just tired? I'll usually start up my hills tacking about 1650 at 60 in 8 direct, and end up dropping to 7 direct and topping the hill at about 45, there's a couple hills in a couple areas with lower limits of course that I'll start a pull in 5th direct tacking 2000 and watching the tach fall all the way down to 1300 before topping the hill. Anybody from Nebraska might know the grade of the hill I reckon its about 9% or so its the Bellevue hill as you come off the road that runs south of offut AFB to cross the bridge over to 370 to run over to CB IA. Also the Union NE hill on 34 headed east is the same way start up throttled at 2000 and just watch the tach fall off. Can't tell you what the EGTs are pyro never climbs above about 800 but it also seems to run pretty hot its nothing to see 200 on the temp gauge after pulling a hill fan will kick on till about 190 and then I manually kick the fan on when I hit the flat land to cool it off back down to the center line (99 Pete is what its in)

As I said idk much about a Cummins but it seems like a 525 would pull a lot better than what it does, also seems to he a rattle on the back end of the motor somewhere can hear it in the cab when she's under load and in the throttle not sure if it needs the overhead run on it or what its a farmers truck that I'm working for at the moment, and it just don't seem like she's running and pulling the way it should. Thanks in advance for any advise I appreciate it.