the 4.2 is a stroked 3.9 with the same piston bore, so technically you could take the pistons,rods and crank (crank journal size is a commom share amongst ALL rover/buick based engines) and dump it into the 3.9 block to convert it to 4.2 but by using the 3.9 you get a set of cross bolted mains which then lends it self to supercharging,turboing or other major powwer upgrades with out worries.
before the crossbolt, england attempted to build a turbodiesel v8 based on the old 3.5 but the compression ratio tore the threads out of the main caps on the block.
I guess they wanted to emulate the gm 6.2 which was a gas 350 converted to diesel and the compresion would tear thru the rod and main bearings locking the engine or throwing rods.
personally I would stroke a 3.9 just because of the cross bolt feature.
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