MK4 to MK6
 

Is it possible to replace a Mk4 big block with a Mk6 without a major rework? What's required besides a different oil pan ?

If you're talking a longblock swap, then the fuel pump is your only issue. The other different parts will already be on the longblock.

crank is different but you can get an adapter for the gen VI block to use a MKIV crank otherwise the flywheel/harmonic damper will also be different if you change to a gen VI crank, alternator/PS brackets need holes redrilled (bolts are larger and offset about 1/8" so you can redrill the hole off center with a drill press to fit) gen VI is missing the hole for the front mounting bolt of the starter motor if the mkIV block has the old large starter( there is a 1/2"npt pipe thread plug in this location so you can tap a 1/2 " plug for the 3/8 bolt if you want to use the original large starter), the seawater pump will need to be shimmed to line up with the serpentine/ V belts, timing cover is different, Different oil pan as you knew and different dip stick

nothing major just a few idiosycracies