Dummy fitted an old RB30 block last night using the 200ZR mounts. It looks like the mounts are going to work with a bit of modification, thanks Sco_Aus for the tip
As you can see from the pic below they will sit the engine in about the same spot in the engine bay as the L series was.
The 240Z mounting points on the cross member are slightly further apart than in a 200ZR so it needs a spacer plate between the mounts and the block to line up nicely. The plan is to weld on a 5mm spacer plate to the face of each mount and drill some new holes to sit the engine about 5mm further back, 15mm down on the passenger side, and 25mm down on the drivers side (it's currently on more of an angle than it would sit in a Skyline). This should sit the engine spot on where we want it. I'll post up some pics of the mounts when we're done modifying.