By shifting between 2nd and 3rd is an ordeal, do you mean the gears grind or it is hard to physically shift the lever?

Does the tranny shift smoothly when you match engine speed to road speed?

Does the clutch slip? Is it hard to get into 1st gear sitting still?

I'm asking all these questions because a common failure is the synchro gears in the tranny. Clutch failure is pretty unusual.

My wife is driving a '72 250C which I installed an engine/trans unit I pulled from her old '71 220. I bought the original car in '95 and it STILL has the original clutch in it. I had to change the transmission because the synchros went bad. The clutch lasted through two sons driving the car to high school and is still going strong.