[silverc4s]

There is an early (pre mag case) fan housing / fan assembly that is NOT compatible with later car hardware. Don't ask me how I learned this...
It is a cast aluminum housing, NOT magnesium, and it is a couple of cm larger in diameter, requiring a special fan as well. Perhaps someone sold you one of these housings. If so you should be able to swap even with an early car restorer for the housing you need, as these are now pretty rare, I believe...
Good luck, don't destroy the one you have making it fit, only to find a simpler solution.
PS In your picture it DOES look like aluminum to me..