Why not Jez ! If you are after some more power this would be the easiest way to do so.
It is up to you if you fun factor over concourse originality and everyone is different.
The higher compression pistons would make a considerable difference in the running but as Tony said, for reliability it would be worth giving the engine a full strip and check over for reliability purposes.
At the original rolling road the car that had the highest HP had an oem volvo B28E block with standard exhaust and that had 154hp (if I remember right) compared to others that were about 137-140. (Ask Dan what his original rolling road result was for comparison considering that he has the performance exhaust on too).
If you do go ahead with the swap I am sure you could get some money back for your original block as it is a known running example.
Could you listen to the engine run before you remove it?
(PS DarrenC, have you got over your heart attack after hearing about NON-OEM Custom Dirty Second hand parts about to be fitted
)