Just did this last month with the Catalyst bodywork. Came out very nice. Just drilled the two forward holes that are under the rear of the seat, and bolted it loosely into place.
Then, drilled very tiny holes at the rear (underneath) mounting points and looked through them to sight the holes. Then enlarged them enough to fit two bolts (same as the stock upper bolts), using a Unibit.
Got a couple clip nuts and installed them over the subframe tabs so that I could install the bolts from underneath.
BTW, it helps a lot to remove the brackets on top of the subframe that mount the stock tailsection, as well as the latch mechanism. Makes it much easier to slide the tailsection on.
To get the seat to mount, I used my die-grinder to notch the front of the tail in order to be able to install the seat after the tail is in place.
I can take pics if it would help. Lemme know.