Interstates started getting built in Eisenhower time frame. So 50's!
But as noted, my 4.3 V6 has 50 more HP, down 35 lb ft of torque in motor, my 88&89 454's. 6 gears instead of three. I pull my equipment trailer with mini excavator better and faster with my 2014 2500. A bit better than my 2000 C2500 5.7 4 sp 4l80 trans.
My son towed his dodge conversion van from Wenatchee over Stevens pas, which is way harder pull than I90 with his 05, 4.8 V8, 3.42 gears, 4" lift and 33.5" tires. He survived. A bit slow.
Enjoy.
Marty