Hi guys !

Still a bit new to this world, I have few questions regarding the following subjects:

1)Diff

2)Brake booster

My car is a 245L from 1975 with a B20A mated to a M45. Around 125'000 miles / 200'000km

1) Rear diff:
My rear diff has in my opinion a short ratio. In 4th gear, my engine is running high in the rpm (can't give a number since I don't have a tacho) at 90km/h // 55mph. If I want to overtake, I need to go to a 65mph / 100km/h and the engine is really high in the rpm.
So, I need your expertise on that, should I swap a m47 instead (I know, that I have to find an early m47 because of the speedometer).
Or should I think about changing the ratio in my diff.

2) brake booster:
My brakes are working fine, however when hot, the brake booster makes a ticking noise when the brake pedal is not touch and when stationary. If i bend the vacuum pipe that links the brake booster valve to the engine, the ticking noise goes away. I read a lot of comment saying that the valve is the first thing to change but, in my case I am unsure if it's really useful to swap a new valve. Before installing a new brake booster, I would like to be sure that I do the right thing.

Thanks for your time !

Regards,
Tobias