I have a Bushy Shorty 16" can I convert to a Dissy?

Yes it is possible, but you have some work ahead of you.  Refer to this picture in the link provided when reading the description.

www.bushmaster.com/shopping/barrel-assemblies/abbl-16dm4a.asp

First you need another gas block of some type, a clamp on type would make your job much easier.  I personally would hate to try to drill/ream holes and install taper pins in the "fake" gas block, but it can be done.

Second, you will probably have to shave your existing gas block so it will fit under the handguards, but still maintain its function.

You need to get a full length handguard set with handguard cap.  Install these like you would the regular handguards on the barrel.  This will tell you where to mount your "fake" gas block.  To keep your handguards from spinning when installed, a proper sized roll pin or a screw/bolt (screw/bolt would require drilling/tapping gas hole in "fake" gas block) in the "fake" gas block will align with the gas tube in the handguard cap to keep them in place.

Please fill in any blanks I left if you know of any, but I've seen it done and doesn't seem to be that hard to do.

Good luck!