P38 4.0 short block merged into 4.2 RR engine

Well I started a straightforward overhaul on my 1993 Range Rover 4.2 EFi (U.S. spec Lucas). An important clue was the former owner telling me he had the dealer "replace the block with a new one" after a crack developed in the original. After I got everything apart and started measuring I realized this is not a RR 4.2 crank or pistons. In looking at the part numbers I think that somebody (about the year 2000 or so) bought a new 4.0 short block assembly (To an American such as me "short block" means the block plus crank, con rods, pistons, camshaft, and lifters), then bolted back on the original 4.2 heads and induction system. I have proceeded to run it fine to 261,000 miles - though I did this by chucking the Lucas EFI and distributor and replacing with an Edelbock carb and Mallory Unilite.

Does anyone know whether using the 4.0 block with 4.2 heads was common? It has run fine until getting recently tired, but I am now wondering what this swap may have done to the original compression ratio.

Heads are identical from 3.5 to 4.6.

Apart from the change from 14 bolt to 10 bolt which was a long overdue upgrade.