I had a 81 mercedes 240d stick shift. It was my dads and 3 other people before I got it with 470 thousand miles on all original parts.
I drove it like him, which turned out we were lugging the hell out of it. I too later found out it cant be over reved and not to be afriad to push the gas petal to the floor. I did however adjust a screw on the injector pump to put in more fuel, which made it smoke under load and installed a kn air filter.
Before I had the junkyard tow it away I did just that, I picked a gear, took off and did not shift. The engine started to howl like a typical v8 at 4 grand with flow master mufflers. Being use to the clattering the roaring noise took some use to.
I think not having a fan clutch caused more noise from under the hood too.
Man, it wasnt a bad driving car after that change in shifting. I am pretty sure I wasnt getting 32mpg. I basically drove to highway speeds in 2nd and third gear and treated 4 as if it was an over drive gear. I shifted out of first at 40 mph, 2nd at 50, 3rd at 65.
Still if he cant past 50 mph in the stock configuration I am sure any mods will push the engine beyond the designers original design and shorten its life or cause some extra mods needed.