I put those same S.D. Dynasonics in my Duo Jet a year ago and consider it one of the best things I've ever done to/with a guitar. The sound is phenominal!
The chiseling can be done with a dremmel tool. If you have decent wood working skills, it should not be a big deal. If not, get some help.
My S.D. Dynas are about 1/8" WIDER than filtertrons I have on another Gretsch (wider = distance running perpendicular to the strings) So that will take some cutting on the top (1/16" each side. However, they are more narrow than the filtertrons (1/4"??) (narrow = distance running parallel with the strings) so you'll probably need cover rings specific to this application. If you call S.D. people, they can probably help you with that.
Good luck -- sounds like you are on your way to a most unique and awesome sounding guitar!