I don't know the details, but there was a change to the regulations regarding depressurization after the 747-400 was certified that basically limits newly certified aircraft to 43k (43,100 ft. if you want to get anal about it).
Regarding the 767, it depends on the engines. I went on a flight test on the 767 AOA (
https://www.globalsecurity.org/space/systems/aoa.htm ) many moons ago where we cruised around for hours at 43k - but it had JT9D-7R4 engines (~50k rating) and to get to those altitudes they had to strip out most of the interior to save weight (what I really remember is that they'd stripped out much of the insulation and it was really, really cold in the cabin).
The later versions of the 767-200 with CF6-80C2 or PW4000 engines could be rated as high as 60k and could easily make it to 43k if they weren't too heavy.