Have you ever seen a Vg30det or Vq30det in person? The Vg30det isn't much bigger than the Vg30et. Of course the intake manifold and heads are bigger since it's DOHC but it isn't that much larger. However, the vq30det has an engine "cover" that makes it quite a bit larger. Of course, this cover can be removed and not used though, the intake manifold design is still a bit larger. Actually, I wouldn't be surprised if the Vg30det and VQ30det, were very similar in size.
Anyways, whether or not they are the same size, or one larger than the other, check out this pic of a Datsun 510 w/ a Vg30det. If that fits the 510 engine bay, I'm pretty sure you could get one in a 240sx!

Vg30det