Firewall, indeed MERS I have forgotten or our CY never used that term, anyhow, I had my share as well.
Like around Delhi You were always required to climb to an altitude, where the old lady was on, or a tad over max. But we had no FMS so the F/E (Flight Engineer for You magenta generation, the one in the 2nd row who dared to make jokes about captains) had to look at the charts. When it took long You knew the answer was biased because otherwise he might had to start being anxious about fuel levels, hours later...
So You crept to the assigned altitude and deliberately overshot it by some 300 feet (no mode C then...), then V/Sed it back to the required alt, keeping climb power, and getting it "on the step" as those old warriors called it then. No big deal.
Cruising was all w/o ATS then, speed monitoring during cruise was an issue not to forget. But at high altitudes the throttles were close to max CRZ anyhow.
It got dangerous however when we started mixed flying with the -300 who did have an ATS. After a few legs with the -300 You completely forgot to watch the speed in CRZ if an occasional 200 was in one of the legs...
Yes, years later and in CMD myself on the -400 doing GUA-MEX at a very light load I also tried FL451. Also for the heck of it. We left a clear and smooth 410 or so and levelled 451, and then all hell broke loose with unexpected heavy turbulence, and we went down in hurry again!. So my FL451 experience has been limited tot less than one minute!