External Zips come apart really (almost too) easily. There are slots in the sides (two on one side, one on the other) that have to have a thin flat-head screw driver pressed in to release spring clips. The front kind of tips forward to let the top off. Once the top is off, however, a lot of the parts are held in by gravity, so don't flip it over to look at it from a different angle with the top off!

The only SCSI out is via the 25-pin D connectors (no 50 pin ribbon in there), so you'll have to do some cable hacking.

It really might be easier to leave the case intact and position it behind your new slot with a (much nicer than) erector set arrangement.