The 320 is an M50 too.
The M50 came in 325 and 320s, and after about 1996, the newer updated M52 came out.
There was a 320 M52 but fairly rare, the 323 was just a 170bhp 2.5, and the 328 was 2.8 litres, but held back to just about the old 325.
There is differences between the 320 and 325 intake manifolds, but the port in the head are different sizes. The bigger 325 one will fit, but it'll have a step just as the air enters the cylinders to duct down to the smaller port size.