In a nut shell. The 3.5 and the 3.9 used the same crank. 3.9 had a larger bore. The 4.2 used the same bore as the 3.9 but a longer stroke crank. The 4.0 has the same bore and stroke as the earlier 3.9, but internally it is a much different engine. The 4.6 has the same bore as the 4.0 but a longer stroke crank. The later 4.0 and 4.6 are better engines because of the larger journal sizes on the crank and because the mains are cross bolted. For the most part, externals will bolt on and cross between the different engines. The later engines use a different cam and cam retainer, but can use the older cams. The crank snouts are different lengths. You need to run a spacer to make the older multi pulley balancer work on the newer cranks. The newer engines used a serpentine setup. Lots more subtle differences, so you will need to do some reading up before you get too deep into it. The expense in these engines is not in acquiring the engine, it's in acquiring all of the hard to find parts that bolt to it. The bolt ons that come out of the Range Rovers that the engines are sourced from are virtually useless in a sports car install. You will need to source proper fitting oil pan $300, timing cover $200, bell housing and shifter components $300, clutch $200, flywheel $200, intake and carb $500, headers $300, you see where I'm going.