Swapped Motor?

I have an 80 528i, which like many other older BMWs, overheats at idle or low speed operation. I think I've located the problem to a defective fan clutch, and was all set to order one when it occured to me that the original motor had been swapped out for another, according to the previous owner. I'm not sure if it's an OEM replacement, or something from another BMW series, but on the left side of the block it states: 3.0/3.3.

I'm not certain, but wasn't the original engine a 2.7 or 2.8?

In any case, there isn't a large 36mm nut between the fan clutch and pulley as I expected, but there is a 13-15mm nut at the front of the fan that I can't get to it without removing the radiator.

Any ideas out there to point me in the right direction?

Xotie

the 1980 528i i have has and came with the 2.8 inline 6