Interesting question. Might take some digging to find an answer. The hydraulic heads never came to us in the US on the Volvo engines but we did get them in the 4 cylinder VW versions. Never tried mixing the setups with those firsthand but I believe they did allow some interchange if you knew what you were doing. Seems like maybe the hydraulic heads needed an extra oil feed or return from the block so swapping a hydro head onto a mechanical bottom end was a no-no, but you could use a mechanical head on a hydraulic block without problems on those if my memory serves. Might have needed to choose the right head gasket too.
On the other hand the 4 and 6 cylinder engines have enough differences that info from one might not be correct for the other. One way to be more sure may be to compare the part numbers or casting numbers for the Volvo hydraulic blocks vs mechanical blocks, if you have access to both. If the blocks have the same number then you probably could use whatever combination you can get your hands on.
If I had to make a guess I would say you could put the solid head on that later hydraulic block and it would work out OK, if the car's owner can live with adding occasional valve clearance checking and adjustment to their maintenance schedule.
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86 745 D24T/ZF 345k lifted 2.5"
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