Cyllinder Head ID Questions

I spent yesterday helping a mate remove the head from his '98 E39 RHD. It had been diagnosed with gasses in the coolant and when the head finally came off there was a heap of oil over the face of the block between the 3rd and 4th cyllinder. We even broke the ends off two socket extension bars trying to remove one of the bolts. This vehicle is supposed to be a 528i but the head casting says 2.5, soooo... does this mean one of the following?

It is actually a 2.5 engine that has been changed out ?

Could the heads between the two different capacity engines be interchangeable?

Is there a way of tracking the actual head or engine via the stamped numbers?

Other relevant factors include, this engine only has single Vanos and has the plastic valve cover on it unlike the E34. As you would imagine these questions need to be answered before new parts are ordered.

Cheers

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