[Lynn Eades]

Petr,Do you have the original fan?
It takes a lot of driving and I think both belts will be required.
The fan mounts on a fan extension which has a fibre coupling to drive the generator. No side load is put on the generator bearings.
Usually double pulley water pumps have a bearing in them. Single pulley pumps are bushed.
Your generator is not the right one. It should have a thread on the drive end to take a keyed drive dog.
If you need photos I can look for them.

Edit. The fan needs to be positioned in the fan shroud to work properly. The fan extension does this. you should aim to fit all the shrouds to help the difficult task of moving cool air through the radiator etc.
I have seen carriers that overheat because parts are left out.