ok here's what I have to base this from, it's not the same as yours but simmiler.

78 F250 4x4 C6 203. the motor is a 71 lincon 460 .030 over with 75 heads and intake. Gears are 3.50 ish (not actualy sure a 9" is 3.50 but I don't know what a D60 would be. the front D44 is 3.54) The load I'm basing this on is my camper and utility trailer + atc. Combined weight was just under 6 tons (11,780lbs) The route traveled was the hope princton.

Temp 170 on the flats up to 200 on the hills with my foot stuffed into the floor.

2400 is a little low for 4th with a load and hill. It should pull up from there tho. If I jump on mine under 2550 2600 it bogges for a bit untill it gets up over 2700 and then runs hard up into the 3500 4k range. Drop into seond and it will pull it legs out all the way down to 1600. If you like everything else about this truck dump 4:10's into the pumkins and be happy.

My 80 351/np435/208 and 4:10's ran circles around this auto fed 3.5 geard poo machine. AS soon as I get my Bronco out of the shop the 78 is getting the 4spd/205 combo and 4:10 gears.