Series 3 Bigger alternator

My alternator wasn't working and a friend had an extra alternator which he says came of a Mercedes. It is slightly bigger and is rated 80.
It fits into place but has a bigger pulley in front. Because of that the belt fits in with some difficulty but does fit in. I have 2 concerns:
1 since the pulley has a bigger radius it will rotate less with each rotation of the belt
2 the pulley has shorter walls and 1/3 of thickness of the belt sticks out.
Can I use this alternator?
thanks in advance

It will be ok as a temporary measure. Will not make as much power as it could and will wear belt faster.

The alternator should be OK but at 80A you will be working the belt harder so you need it to fit well. Often the pulleys are the same fit so you may be able to swap them over.. The bigger pulley will reduce the chargeing at low revs which can be a problem if you are often in traffic jams.

If your after a direct swap later on I recommend a Lucas A127, works a charm in my series. Otherwise can you swap the pulley off your old alternator?

I will try re-opening and trying to add the pulley today. Last three days it worked fine and reading of voltage on the battery did not show any problems. I am only worried about the belt so we try to see if I can fit the old pulley into this one.
thanks for your replies

I read somewhere every extra 25a of alternator output costs about 1hp

That's an interesting figure. 25A is around 360w at output volts which is close to 1/2hp electrical power but there's all the losses so 1hp at the engine may not be that far off.