Actually the only thing you have to worry about when trying to swap the DE in, fitment wise, would be the width. It'll be shorter and lower than an inline 4 so you just have to worry about the width being that the VG30DE(TT) is extremely wide.
Now... we know its actually been done in the past so width shouldn't be much of an issue.
One problem you'll likely run into is the KA24's had a front sump oil pan (if I remember correctly). The VG-DE has a rear sump pan stock which will likely cause problems. There might be a slightly chance you could use the front sump oil pan from a VG30E as I'm pretty sure the bolt pattern is the same but the pickup tube might not be able to swap from an E to a DE. I'm working on a project right now that might answer that question but it may be a little while before I'll know.
Like Cody said, the motor mounts will have to be custom as well as tranny mounts as you'll probably be using the Z32 transmission. Transmission wise you may actually contact Mazworx (
www.mazworx.com) as they do have a kit to bolt a Z32 5-speed to an SR in a S13 and S14 chassis. They may be able to provide you with the parts needed simply for the 5-speed into the S13 chassis (driveshaft, shifter, shifter bracket, transmission mount/bracket, etc.) The only thing you'll need to worry about is engine placement due to where the tranny is... though it should still be closer to the middle of the car than the I4 would be, just depends how far the tranny sticks out.
From there you have to deal with the wiring. There should be a few wires in the VG harness that you'll have to splice into the 240 harness. The tach will need to be modified (I believe you can take it apart and adjust for the new RPM signal to read correctly for the V6).
That's all I can think of at the moment. I've still not seen any good documentation on how to do the swap so a lot of it is stuff you'll have to figure out along the way.