Of course most commercial 70’s aircraft would struggle to reach anything like 35000ft. Thicker air=more fuel burn.
Rubbish! 747-200, VC-10 even the 707 have ceilings of over FL400.
It’s modern jet engines, that make the most difference in pollution, high bypass, more efficient, less NOx, quieter.
That’s why BA got rid of the 747-400, inefficient, two more engines to maintain and a much higher fuel cost per passenger than a 787/A350.
The reason why the black soot came out of the jet pipes was that the engines where far less efficient in burning all the jet A1 that went into the engine.
Bit like the old Diesel engines, you never see a euro 6 diesel chucking out soot now (unless you remap it and remove the dpf)