After changing the clutch and installing the summer tire/wheels, I noticed vibrations at 65-75 mph.
I had the tires balanced, but the vibrations still exists.
Shifting in neutral won't change it.
Could it be the drive shaft?
How to narrow down the problem source?

 

you could try switching the "more true" wheels to the front. Make sure there arent any bubbles in the tires. Make sure your lugs are torqued correctly and evenly. Make sure all the wheels have hubcentric rings on them, (cuz it sounds like they are aftermarket). It would be weird that any suspension or alignment changed at the same time, but id start checking that next.

 

Also i hope you aligned your clutch disc properly when you did the install, but im leaning more toward a tire/wheel problem.